BUG: memcmp(): Accessing invalid memory location

When executing fstests' generic/026 test, I hit the following call trace,

[  417.061038] BUG: Unable to handle kernel data access at 0xc00000062ac40000
[  417.062172] Faulting instruction address: 0xc000000000092240
[  417.062242] Oops: Kernel access of bad area, sig: 11 [#1]
[  417.062299] LE SMP NR_CPUS=2048 DEBUG_PAGEALLOC NUMA pSeries
[  417.062366] Modules linked in:
[  417.062401] CPU: 0 PID: 27828 Comm: chacl Not tainted 5.0.0-rc2-next-20190115-00001-g6de6dba64dda #1
[  417.062495] NIP:  c000000000092240 LR: c00000000066a55c CTR: 0000000000000000
[  417.062567] REGS: c00000062c0c3430 TRAP: 0300   Not tainted  (5.0.0-rc2-next-20190115-00001-g6de6dba64dda)
[  417.062660] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000842  XER: 20000000
[  417.062750] CFAR: 00007fff7f3108ac DAR: c00000062ac40000 DSISR: 40000000 IRQMASK: 0
               GPR00: 0000000000000000 c00000062c0c36c0 c0000000017f4c00 c00000000121a660
               GPR04: c00000062ac3fff9 0000000000000004 0000000000000020 00000000275b19c4
               GPR08: 000000000000000c 46494c4500000000 5347495f41434c5f c0000000026073a0
               GPR12: 0000000000000000 c0000000027a0000 0000000000000000 0000000000000000
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
               GPR20: c00000062ea70020 c00000062c0c38d0 0000000000000002 0000000000000002
               GPR24: c00000062ac3ffe8 00000000275b19c4 0000000000000001 c00000062ac30000
               GPR28: c00000062c0c38d0 c00000062ac30050 c00000062ac30058 0000000000000000
[  417.063563] NIP [c000000000092240] memcmp+0x120/0x690
[  417.063635] LR [c00000000066a55c] xfs_attr3_leaf_lookup_int+0x53c/0x5b0
[  417.063709] Call Trace:
[  417.063744] [c00000062c0c36c0] [c00000000066a098] xfs_attr3_leaf_lookup_int+0x78/0x5b0 (unreliable)
[  417.063851] [c00000062c0c3760] [c000000000693f8c] xfs_da3_node_lookup_int+0x32c/0x5a0
[  417.063944] [c00000062c0c3820] [c0000000006634a0] xfs_attr_node_addname+0x170/0x6b0
[  417.064034] [c00000062c0c38b0] [c000000000664ffc] xfs_attr_set+0x2ac/0x340
[  417.064118] [c00000062c0c39a0] [c000000000758d40] __xfs_set_acl+0xf0/0x230
[  417.064190] [c00000062c0c3a00] [c000000000758f50] xfs_set_acl+0xd0/0x160
[  417.064268] [c00000062c0c3aa0] [c0000000004b69b0] set_posix_acl+0xc0/0x130
[  417.064339] [c00000062c0c3ae0] [c0000000004b6a88] posix_acl_xattr_set+0x68/0x110
[  417.064412] [c00000062c0c3b20] [c0000000004532d4] __vfs_setxattr+0xa4/0x110
[  417.064485] [c00000062c0c3b80] [c000000000454c2c] __vfs_setxattr_noperm+0xac/0x240
[  417.064566] [c00000062c0c3bd0] [c000000000454ee8] vfs_setxattr+0x128/0x130
[  417.064638] [c00000062c0c3c30] [c000000000455138] setxattr+0x248/0x600
[  417.064710] [c00000062c0c3d90] [c000000000455738] path_setxattr+0x108/0x120
[  417.064785] [c00000062c0c3e00] [c000000000455778] sys_setxattr+0x28/0x40
[  417.064858] [c00000062c0c3e20] [c00000000000bae4] system_call+0x5c/0x70
[  417.064930] Instruction dump:
[  417.064964] 7d201c28 7d402428 7c295040 38630008 38840008 408201f0 4200ffe8 2c050000
[  417.065051] 4182ff6c 20c50008 54c61838 7d201c28 <7d402428> 7d293436 7d4a3436 7c295040
[  417.065150] ---[ end trace 0d060411b5e3741b ]---


Both the memory locations passed to memcmp() had "SGI_ACL_FILE" and len
argument of memcmp() was set to 12. s1 argument of memcmp() had the value
0x00000000f4af0485, while s2 argument had the value 0x00000000ce9e316f.

The following is the code path within memcmp() that gets executed for the
above mentioned values,

- Since len (i.e. 12) is greater than 7, we branch to .Lno_short.
- We then prefetch the contents of r3 & r4 and branch to
  .Ldiffoffset_8bytes_make_align_start.
- Under .Ldiffoffset_novmx_cmp, Since r3 is unaligned we end up comparing
  "SGI" part of the string. r3's value is then aligned. r4's value is
  incremented by 3. For comparing the remaining 9 bytes, we jump to
  .Lcmp_lt32bytes.
- Here, 8 bytes of the remaining 9 bytes are compared and execution moves to
  .Lcmp_rest_lt8bytes.
- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
  value and hence ends up accessing the "next" double word. The "next" double
  word happens to occur after the last page mapped into the kernel's address
  space and hence this leads to the previously listed oops.
  
-- 
chandan

On Thu, 2019-01-24 at 19:48 +0530, Chandan Rajendra wrote:

This is interesting ... should we mark the last page of any piece of
mapped linear mapping as reserved to avoid that sort of issue ?

Nick ? Aneesh ?

Cheers,
Ben.

Le 25/01/2019 à 01:55, Benjamin Herrenschmidt a écrit :

On Thu, 2019-01-24 at 19:48 +0530, Chandan Rajendra wrote:

quoted

- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
   value and hence ends up accessing the "next" double word. The "next" double
   word happens to occur after the last page mapped into the kernel's address
   space and hence this leads to the previously listed oops.
   

This is interesting ... should we mark the last page of any piece of
mapped linear mapping as reserved to avoid that sort of issue ?

Or revert to a normal comparison once remaining length is < 8 and r4 in unaligned ?

Christophe

Nick ? Aneesh ?

Cheers,
Ben.

Adding Simon who wrote the code.

Chandan Rajendra [off-list ref] writes:

When executing fstests' generic/026 test, I hit the following call trace,

[  417.061038] BUG: Unable to handle kernel data access at 0xc00000062ac40000
[  417.062172] Faulting instruction address: 0xc000000000092240
[  417.062242] Oops: Kernel access of bad area, sig: 11 [#1]
[  417.062299] LE SMP NR_CPUS=2048 DEBUG_PAGEALLOC NUMA pSeries
[  417.062366] Modules linked in:
[  417.062401] CPU: 0 PID: 27828 Comm: chacl Not tainted 5.0.0-rc2-next-20190115-00001-g6de6dba64dda #1
[  417.062495] NIP:  c000000000092240 LR: c00000000066a55c CTR: 0000000000000000
[  417.062567] REGS: c00000062c0c3430 TRAP: 0300   Not tainted  (5.0.0-rc2-next-20190115-00001-g6de6dba64dda)
[  417.062660] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000842  XER: 20000000
[  417.062750] CFAR: 00007fff7f3108ac DAR: c00000062ac40000 DSISR: 40000000 IRQMASK: 0
               GPR00: 0000000000000000 c00000062c0c36c0 c0000000017f4c00 c00000000121a660
               GPR04: c00000062ac3fff9 0000000000000004 0000000000000020 00000000275b19c4
               GPR08: 000000000000000c 46494c4500000000 5347495f41434c5f c0000000026073a0
               GPR12: 0000000000000000 c0000000027a0000 0000000000000000 0000000000000000
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
               GPR20: c00000062ea70020 c00000062c0c38d0 0000000000000002 0000000000000002
               GPR24: c00000062ac3ffe8 00000000275b19c4 0000000000000001 c00000062ac30000
               GPR28: c00000062c0c38d0 c00000062ac30050 c00000062ac30058 0000000000000000
[  417.063563] NIP [c000000000092240] memcmp+0x120/0x690
[  417.063635] LR [c00000000066a55c] xfs_attr3_leaf_lookup_int+0x53c/0x5b0
[  417.063709] Call Trace:
[  417.063744] [c00000062c0c36c0] [c00000000066a098] xfs_attr3_leaf_lookup_int+0x78/0x5b0 (unreliable)
[  417.063851] [c00000062c0c3760] [c000000000693f8c] xfs_da3_node_lookup_int+0x32c/0x5a0
[  417.063944] [c00000062c0c3820] [c0000000006634a0] xfs_attr_node_addname+0x170/0x6b0
[  417.064034] [c00000062c0c38b0] [c000000000664ffc] xfs_attr_set+0x2ac/0x340
[  417.064118] [c00000062c0c39a0] [c000000000758d40] __xfs_set_acl+0xf0/0x230
[  417.064190] [c00000062c0c3a00] [c000000000758f50] xfs_set_acl+0xd0/0x160
[  417.064268] [c00000062c0c3aa0] [c0000000004b69b0] set_posix_acl+0xc0/0x130
[  417.064339] [c00000062c0c3ae0] [c0000000004b6a88] posix_acl_xattr_set+0x68/0x110
[  417.064412] [c00000062c0c3b20] [c0000000004532d4] __vfs_setxattr+0xa4/0x110
[  417.064485] [c00000062c0c3b80] [c000000000454c2c] __vfs_setxattr_noperm+0xac/0x240
[  417.064566] [c00000062c0c3bd0] [c000000000454ee8] vfs_setxattr+0x128/0x130
[  417.064638] [c00000062c0c3c30] [c000000000455138] setxattr+0x248/0x600
[  417.064710] [c00000062c0c3d90] [c000000000455738] path_setxattr+0x108/0x120
[  417.064785] [c00000062c0c3e00] [c000000000455778] sys_setxattr+0x28/0x40
[  417.064858] [c00000062c0c3e20] [c00000000000bae4] system_call+0x5c/0x70
[  417.064930] Instruction dump:
[  417.064964] 7d201c28 7d402428 7c295040 38630008 38840008 408201f0 4200ffe8 2c050000
[  417.065051] 4182ff6c 20c50008 54c61838 7d201c28 <7d402428> 7d293436 7d4a3436 7c295040
[  417.065150] ---[ end trace 0d060411b5e3741b ]---


Both the memory locations passed to memcmp() had "SGI_ACL_FILE" and len
argument of memcmp() was set to 12. s1 argument of memcmp() had the value
0x00000000f4af0485, while s2 argument had the value 0x00000000ce9e316f.

The following is the code path within memcmp() that gets executed for the
above mentioned values,

- Since len (i.e. 12) is greater than 7, we branch to .Lno_short.
- We then prefetch the contents of r3 & r4 and branch to
  .Ldiffoffset_8bytes_make_align_start.
- Under .Ldiffoffset_novmx_cmp, Since r3 is unaligned we end up comparing
  "SGI" part of the string. r3's value is then aligned. r4's value is
  incremented by 3. For comparing the remaining 9 bytes, we jump to
  .Lcmp_lt32bytes.
- Here, 8 bytes of the remaining 9 bytes are compared and execution moves to
  .Lcmp_rest_lt8bytes.
- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
  value and hence ends up accessing the "next" double word. The "next" double
  word happens to occur after the last page mapped into the kernel's address
  space and hence this leads to the previously listed oops.

Thanks for the analysis.

This is just a bug, we can't read past the end of the source or dest.

We have a selftest for memcmp, but clearly it doesn't exercise this
case. Here's a patch to try and trip it:

diff --git a/tools/testing/selftests/powerpc/stringloops/memcmp.c b/tools/testing/selftests/powerpc/stringloops/memcmp.c
index b1fa7546957f..edca3abb6ecf 100644
--- a/tools/testing/selftests/powerpc/stringloops/memcmp.c
+++ b/tools/testing/selftests/powerpc/stringloops/memcmp.c

@@ -2,6 +2,7 @@
 #include <malloc.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/mman.h>
 #include <time.h>
 #include "utils.h"
 

@@ -68,25 +69,24 @@ static void test_one(char *s1, char *s2, unsigned long max_offset,
 
 static int testcase(bool islarge)
 {
-	char *s1;
-	char *s2;
-	unsigned long i;
-
-	unsigned long comp_size = (islarge ? LARGE_SIZE : SIZE);
-	unsigned long alloc_size = comp_size + MAX_OFFSET_DIFF_S1_S2;
-	int iterations = islarge ? LARGE_ITERATIONS : ITERATIONS;
-
-	s1 = memalign(128, alloc_size);
-	if (!s1) {
-		perror("memalign");
-		exit(1);
-	}
+	unsigned long i, comp_size, alloc_size, map_chunk;
+	char *p, *s1, *s2;
+	int iterations;
 
-	s2 = memalign(128, alloc_size);
-	if (!s2) {
-		perror("memalign");
-		exit(1);
-	}
+	comp_size = (islarge ? LARGE_SIZE : SIZE);
+	alloc_size = comp_size + MAX_OFFSET_DIFF_S1_S2;
+	iterations = islarge ? LARGE_ITERATIONS : ITERATIONS;
+
+	map_chunk = 64 * 1024;
+	p = mmap(NULL, 4 * map_chunk, PROT_READ | PROT_WRITE,
+		 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+	FAIL_IF(p == MAP_FAILED);
+
+	s1 = p + map_chunk - alloc_size;
+	s2 = p + 3 * map_chunk - alloc_size;
+
+	munmap(p + map_chunk, map_chunk);
+	munmap(p + 3 * map_chunk, map_chunk);
 
 	srandom(time(0));
 

cheers

Michael Ellerman [off-list ref] writes:

Adding Simon who wrote the code.

Chandan Rajendra [off-list ref] writes:

quoted

When executing fstests' generic/026 test, I hit the following call trace,

[  417.061038] BUG: Unable to handle kernel data access at 0xc00000062ac40000
[  417.062172] Faulting instruction address: 0xc000000000092240
[  417.062242] Oops: Kernel access of bad area, sig: 11 [#1]
[  417.062299] LE SMP NR_CPUS=2048 DEBUG_PAGEALLOC NUMA pSeries
[  417.062366] Modules linked in:
[  417.062401] CPU: 0 PID: 27828 Comm: chacl Not tainted 5.0.0-rc2-next-20190115-00001-g6de6dba64dda #1
[  417.062495] NIP:  c000000000092240 LR: c00000000066a55c CTR: 0000000000000000
[  417.062567] REGS: c00000062c0c3430 TRAP: 0300   Not tainted  (5.0.0-rc2-next-20190115-00001-g6de6dba64dda)
[  417.062660] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000842  XER: 20000000
[  417.062750] CFAR: 00007fff7f3108ac DAR: c00000062ac40000 DSISR: 40000000 IRQMASK: 0
               GPR00: 0000000000000000 c00000062c0c36c0 c0000000017f4c00 c00000000121a660
               GPR04: c00000062ac3fff9 0000000000000004 0000000000000020 00000000275b19c4
               GPR08: 000000000000000c 46494c4500000000 5347495f41434c5f c0000000026073a0
               GPR12: 0000000000000000 c0000000027a0000 0000000000000000 0000000000000000
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
               GPR20: c00000062ea70020 c00000062c0c38d0 0000000000000002 0000000000000002
               GPR24: c00000062ac3ffe8 00000000275b19c4 0000000000000001 c00000062ac30000
               GPR28: c00000062c0c38d0 c00000062ac30050 c00000062ac30058 0000000000000000
[  417.063563] NIP [c000000000092240] memcmp+0x120/0x690
[  417.063635] LR [c00000000066a55c] xfs_attr3_leaf_lookup_int+0x53c/0x5b0
[  417.063709] Call Trace:
[  417.063744] [c00000062c0c36c0] [c00000000066a098] xfs_attr3_leaf_lookup_int+0x78/0x5b0 (unreliable)
[  417.063851] [c00000062c0c3760] [c000000000693f8c] xfs_da3_node_lookup_int+0x32c/0x5a0
[  417.063944] [c00000062c0c3820] [c0000000006634a0] xfs_attr_node_addname+0x170/0x6b0
[  417.064034] [c00000062c0c38b0] [c000000000664ffc] xfs_attr_set+0x2ac/0x340
[  417.064118] [c00000062c0c39a0] [c000000000758d40] __xfs_set_acl+0xf0/0x230
[  417.064190] [c00000062c0c3a00] [c000000000758f50] xfs_set_acl+0xd0/0x160
[  417.064268] [c00000062c0c3aa0] [c0000000004b69b0] set_posix_acl+0xc0/0x130
[  417.064339] [c00000062c0c3ae0] [c0000000004b6a88] posix_acl_xattr_set+0x68/0x110
[  417.064412] [c00000062c0c3b20] [c0000000004532d4] __vfs_setxattr+0xa4/0x110
[  417.064485] [c00000062c0c3b80] [c000000000454c2c] __vfs_setxattr_noperm+0xac/0x240
[  417.064566] [c00000062c0c3bd0] [c000000000454ee8] vfs_setxattr+0x128/0x130
[  417.064638] [c00000062c0c3c30] [c000000000455138] setxattr+0x248/0x600
[  417.064710] [c00000062c0c3d90] [c000000000455738] path_setxattr+0x108/0x120
[  417.064785] [c00000062c0c3e00] [c000000000455778] sys_setxattr+0x28/0x40
[  417.064858] [c00000062c0c3e20] [c00000000000bae4] system_call+0x5c/0x70
[  417.064930] Instruction dump:
[  417.064964] 7d201c28 7d402428 7c295040 38630008 38840008 408201f0 4200ffe8 2c050000
[  417.065051] 4182ff6c 20c50008 54c61838 7d201c28 <7d402428> 7d293436 7d4a3436 7c295040
[  417.065150] ---[ end trace 0d060411b5e3741b ]---


Both the memory locations passed to memcmp() had "SGI_ACL_FILE" and len
argument of memcmp() was set to 12. s1 argument of memcmp() had the value
0x00000000f4af0485, while s2 argument had the value 0x00000000ce9e316f.

The following is the code path within memcmp() that gets executed for the
above mentioned values,

- Since len (i.e. 12) is greater than 7, we branch to .Lno_short.
- We then prefetch the contents of r3 & r4 and branch to
  .Ldiffoffset_8bytes_make_align_start.
- Under .Ldiffoffset_novmx_cmp, Since r3 is unaligned we end up comparing
  "SGI" part of the string. r3's value is then aligned. r4's value is
  incremented by 3. For comparing the remaining 9 bytes, we jump to
  .Lcmp_lt32bytes.
- Here, 8 bytes of the remaining 9 bytes are compared and execution moves to
  .Lcmp_rest_lt8bytes.
- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
  value and hence ends up accessing the "next" double word. The "next" double
  word happens to occur after the last page mapped into the kernel's address
  space and hence this leads to the previously listed oops.

Thanks for the analysis.

This is just a bug, we can't read past the end of the source or dest.

How about this, works for me.

cheers

diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S
index 844d8e774492..2a302158cb53 100644
--- a/arch/powerpc/lib/memcmp_64.S
+++ b/arch/powerpc/lib/memcmp_64.S

@@ -215,20 +215,29 @@ _GLOBAL_TOC(memcmp)
 	beq	.Lzero
 
 .Lcmp_rest_lt8bytes:
-	/* Here we have only less than 8 bytes to compare with. at least s1
-	 * Address is aligned with 8 bytes.
-	 * The next double words are load and shift right with appropriate
-	 * bits.
+	/*
+	 * Here we have less than 8 bytes left to compare with. We mustn't read
+	 * past the end of either source or dest.
 	 */
-	subfic  r6,r5,8
-	slwi	r6,r6,3
-	LD	rA,0,r3
-	LD	rB,0,r4
-	srd	rA,rA,r6
-	srd	rB,rB,r6
-	cmpld	cr0,rA,rB
+
+	/* If we have less than 4 bytes, just do byte at a time */
+	cmpwi   cr1, r5, 4
+	blt	cr1, .Lshort
+
+	/* Compare 4 bytes */
+	LW	rA,0,r3
+	LW	rB,0,r4
+	cmpd	cr0,rA,rB
 	bne	cr0,.LcmpAB_lightweight
-	b	.Lzero
+
+	/* If we had exactly 4 bytes left, we're done now */
+	beq	cr1, .Lzero
+
+	/* Otherwise do what ever's left a byte at a time */
+	subi	r5, r5, 4
+	addi	r3, r3, 4
+	addi	r4, r4, 4
+	b	.Lshort
 
 .Lnon_zero:
 	mr	r3,rC

On Friday, February 1, 2019 4:43:52 PM IST Michael Ellerman wrote:

With the above patch, Linux kernel does not end up in oops. Hence,

Tested-by: Chandan Rajendra <redacted>

-- 
chandan

Chandan Rajendra [off-list ref] writes:

On Friday, February 1, 2019 4:43:52 PM IST Michael Ellerman wrote:

quoted

Michael Ellerman [off-list ref] writes:

quoted

Adding Simon who wrote the code.

Chandan Rajendra [off-list ref] writes:

quoted

When executing fstests' generic/026 test, I hit the following call trace,

[  417.061038] BUG: Unable to handle kernel data access at 0xc00000062ac40000
[  417.062172] Faulting instruction address: 0xc000000000092240
[  417.062242] Oops: Kernel access of bad area, sig: 11 [#1]
[  417.062299] LE SMP NR_CPUS=2048 DEBUG_PAGEALLOC NUMA pSeries
[  417.062366] Modules linked in:
[  417.062401] CPU: 0 PID: 27828 Comm: chacl Not tainted 5.0.0-rc2-next-20190115-00001-g6de6dba64dda #1
[  417.062495] NIP:  c000000000092240 LR: c00000000066a55c CTR: 0000000000000000
[  417.062567] REGS: c00000062c0c3430 TRAP: 0300   Not tainted  (5.0.0-rc2-next-20190115-00001-g6de6dba64dda)
[  417.062660] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000842  XER: 20000000
[  417.062750] CFAR: 00007fff7f3108ac DAR: c00000062ac40000 DSISR: 40000000 IRQMASK: 0
               GPR00: 0000000000000000 c00000062c0c36c0 c0000000017f4c00 c00000000121a660
               GPR04: c00000062ac3fff9 0000000000000004 0000000000000020 00000000275b19c4
               GPR08: 000000000000000c 46494c4500000000 5347495f41434c5f c0000000026073a0
               GPR12: 0000000000000000 c0000000027a0000 0000000000000000 0000000000000000
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
               GPR20: c00000062ea70020 c00000062c0c38d0 0000000000000002 0000000000000002
               GPR24: c00000062ac3ffe8 00000000275b19c4 0000000000000001 c00000062ac30000
               GPR28: c00000062c0c38d0 c00000062ac30050 c00000062ac30058 0000000000000000
[  417.063563] NIP [c000000000092240] memcmp+0x120/0x690
[  417.063635] LR [c00000000066a55c] xfs_attr3_leaf_lookup_int+0x53c/0x5b0
[  417.063709] Call Trace:
[  417.063744] [c00000062c0c36c0] [c00000000066a098] xfs_attr3_leaf_lookup_int+0x78/0x5b0 (unreliable)
[  417.063851] [c00000062c0c3760] [c000000000693f8c] xfs_da3_node_lookup_int+0x32c/0x5a0
[  417.063944] [c00000062c0c3820] [c0000000006634a0] xfs_attr_node_addname+0x170/0x6b0
[  417.064034] [c00000062c0c38b0] [c000000000664ffc] xfs_attr_set+0x2ac/0x340
[  417.064118] [c00000062c0c39a0] [c000000000758d40] __xfs_set_acl+0xf0/0x230
[  417.064190] [c00000062c0c3a00] [c000000000758f50] xfs_set_acl+0xd0/0x160
[  417.064268] [c00000062c0c3aa0] [c0000000004b69b0] set_posix_acl+0xc0/0x130
[  417.064339] [c00000062c0c3ae0] [c0000000004b6a88] posix_acl_xattr_set+0x68/0x110
[  417.064412] [c00000062c0c3b20] [c0000000004532d4] __vfs_setxattr+0xa4/0x110
[  417.064485] [c00000062c0c3b80] [c000000000454c2c] __vfs_setxattr_noperm+0xac/0x240
[  417.064566] [c00000062c0c3bd0] [c000000000454ee8] vfs_setxattr+0x128/0x130
[  417.064638] [c00000062c0c3c30] [c000000000455138] setxattr+0x248/0x600
[  417.064710] [c00000062c0c3d90] [c000000000455738] path_setxattr+0x108/0x120
[  417.064785] [c00000062c0c3e00] [c000000000455778] sys_setxattr+0x28/0x40
[  417.064858] [c00000062c0c3e20] [c00000000000bae4] system_call+0x5c/0x70
[  417.064930] Instruction dump:
[  417.064964] 7d201c28 7d402428 7c295040 38630008 38840008 408201f0 4200ffe8 2c050000
[  417.065051] 4182ff6c 20c50008 54c61838 7d201c28 <7d402428> 7d293436 7d4a3436 7c295040
[  417.065150] ---[ end trace 0d060411b5e3741b ]---


Both the memory locations passed to memcmp() had "SGI_ACL_FILE" and len
argument of memcmp() was set to 12. s1 argument of memcmp() had the value
0x00000000f4af0485, while s2 argument had the value 0x00000000ce9e316f.

The following is the code path within memcmp() that gets executed for the
above mentioned values,

- Since len (i.e. 12) is greater than 7, we branch to .Lno_short.
- We then prefetch the contents of r3 & r4 and branch to
  .Ldiffoffset_8bytes_make_align_start.
- Under .Ldiffoffset_novmx_cmp, Since r3 is unaligned we end up comparing
  "SGI" part of the string. r3's value is then aligned. r4's value is
  incremented by 3. For comparing the remaining 9 bytes, we jump to
  .Lcmp_lt32bytes.
- Here, 8 bytes of the remaining 9 bytes are compared and execution moves to
  .Lcmp_rest_lt8bytes.
- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
  value and hence ends up accessing the "next" double word. The "next" double
  word happens to occur after the last page mapped into the kernel's address
  space and hence this leads to the previously listed oops.

Thanks for the analysis.

This is just a bug, we can't read past the end of the source or dest.

How about this, works for me.

cheers

diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S
index 844d8e774492..2a302158cb53 100644
--- a/arch/powerpc/lib/memcmp_64.S
+++ b/arch/powerpc/lib/memcmp_64.S

@@ -215,20 +215,29 @@ _GLOBAL_TOC(memcmp)
 	beq	.Lzero
 
 .Lcmp_rest_lt8bytes:
-	/* Here we have only less than 8 bytes to compare with. at least s1
-	 * Address is aligned with 8 bytes.
-	 * The next double words are load and shift right with appropriate
-	 * bits.
+	/*
+	 * Here we have less than 8 bytes left to compare with. We mustn't read
+	 * past the end of either source or dest.
 	 */
-	subfic  r6,r5,8
-	slwi	r6,r6,3
-	LD	rA,0,r3
-	LD	rB,0,r4
-	srd	rA,rA,r6
-	srd	rB,rB,r6
-	cmpld	cr0,rA,rB
+
+	/* If we have less than 4 bytes, just do byte at a time */
+	cmpwi   cr1, r5, 4
+	blt	cr1, .Lshort
+
+	/* Compare 4 bytes */
+	LW	rA,0,r3
+	LW	rB,0,r4
+	cmpd	cr0,rA,rB
 	bne	cr0,.LcmpAB_lightweight
-	b	.Lzero
+
+	/* If we had exactly 4 bytes left, we're done now */
+	beq	cr1, .Lzero
+
+	/* Otherwise do what ever's left a byte at a time */
+	subi	r5, r5, 4
+	addi	r3, r3, 4
+	addi	r4, r4, 4
+	b	.Lshort
 
 .Lnon_zero:
 	mr	r3,rC

With the above patch, Linux kernel does not end up in oops. Hence,

Tested-by: Chandan Rajendra <redacted>

Thanks.

How many times had you hit the original oops? ie. was it easily
reproducible?

cheers

On Wednesday, February 6, 2019 5:20:04 PM IST Michael Ellerman wrote:

Chandan Rajendra [off-list ref] writes:

quoted

On Friday, February 1, 2019 4:43:52 PM IST Michael Ellerman wrote:

quoted

Michael Ellerman [off-list ref] writes:

quoted

Adding Simon who wrote the code.

Chandan Rajendra [off-list ref] writes:

quoted

When executing fstests' generic/026 test, I hit the following call trace,

[  417.061038] BUG: Unable to handle kernel data access at 0xc00000062ac40000
[  417.062172] Faulting instruction address: 0xc000000000092240
[  417.062242] Oops: Kernel access of bad area, sig: 11 [#1]
[  417.062299] LE SMP NR_CPUS=2048 DEBUG_PAGEALLOC NUMA pSeries
[  417.062366] Modules linked in:
[  417.062401] CPU: 0 PID: 27828 Comm: chacl Not tainted 5.0.0-rc2-next-20190115-00001-g6de6dba64dda #1
[  417.062495] NIP:  c000000000092240 LR: c00000000066a55c CTR: 0000000000000000
[  417.062567] REGS: c00000062c0c3430 TRAP: 0300   Not tainted  (5.0.0-rc2-next-20190115-00001-g6de6dba64dda)
[  417.062660] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000842  XER: 20000000
[  417.062750] CFAR: 00007fff7f3108ac DAR: c00000062ac40000 DSISR: 40000000 IRQMASK: 0
               GPR00: 0000000000000000 c00000062c0c36c0 c0000000017f4c00 c00000000121a660
               GPR04: c00000062ac3fff9 0000000000000004 0000000000000020 00000000275b19c4
               GPR08: 000000000000000c 46494c4500000000 5347495f41434c5f c0000000026073a0
               GPR12: 0000000000000000 c0000000027a0000 0000000000000000 0000000000000000
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
               GPR20: c00000062ea70020 c00000062c0c38d0 0000000000000002 0000000000000002
               GPR24: c00000062ac3ffe8 00000000275b19c4 0000000000000001 c00000062ac30000
               GPR28: c00000062c0c38d0 c00000062ac30050 c00000062ac30058 0000000000000000
[  417.063563] NIP [c000000000092240] memcmp+0x120/0x690
[  417.063635] LR [c00000000066a55c] xfs_attr3_leaf_lookup_int+0x53c/0x5b0
[  417.063709] Call Trace:
[  417.063744] [c00000062c0c36c0] [c00000000066a098] xfs_attr3_leaf_lookup_int+0x78/0x5b0 (unreliable)
[  417.063851] [c00000062c0c3760] [c000000000693f8c] xfs_da3_node_lookup_int+0x32c/0x5a0
[  417.063944] [c00000062c0c3820] [c0000000006634a0] xfs_attr_node_addname+0x170/0x6b0
[  417.064034] [c00000062c0c38b0] [c000000000664ffc] xfs_attr_set+0x2ac/0x340
[  417.064118] [c00000062c0c39a0] [c000000000758d40] __xfs_set_acl+0xf0/0x230
[  417.064190] [c00000062c0c3a00] [c000000000758f50] xfs_set_acl+0xd0/0x160
[  417.064268] [c00000062c0c3aa0] [c0000000004b69b0] set_posix_acl+0xc0/0x130
[  417.064339] [c00000062c0c3ae0] [c0000000004b6a88] posix_acl_xattr_set+0x68/0x110
[  417.064412] [c00000062c0c3b20] [c0000000004532d4] __vfs_setxattr+0xa4/0x110
[  417.064485] [c00000062c0c3b80] [c000000000454c2c] __vfs_setxattr_noperm+0xac/0x240
[  417.064566] [c00000062c0c3bd0] [c000000000454ee8] vfs_setxattr+0x128/0x130
[  417.064638] [c00000062c0c3c30] [c000000000455138] setxattr+0x248/0x600
[  417.064710] [c00000062c0c3d90] [c000000000455738] path_setxattr+0x108/0x120
[  417.064785] [c00000062c0c3e00] [c000000000455778] sys_setxattr+0x28/0x40
[  417.064858] [c00000062c0c3e20] [c00000000000bae4] system_call+0x5c/0x70
[  417.064930] Instruction dump:
[  417.064964] 7d201c28 7d402428 7c295040 38630008 38840008 408201f0 4200ffe8 2c050000
[  417.065051] 4182ff6c 20c50008 54c61838 7d201c28 <7d402428> 7d293436 7d4a3436 7c295040
[  417.065150] ---[ end trace 0d060411b5e3741b ]---


Both the memory locations passed to memcmp() had "SGI_ACL_FILE" and len
argument of memcmp() was set to 12. s1 argument of memcmp() had the value
0x00000000f4af0485, while s2 argument had the value 0x00000000ce9e316f.

The following is the code path within memcmp() that gets executed for the
above mentioned values,

- Since len (i.e. 12) is greater than 7, we branch to .Lno_short.
- We then prefetch the contents of r3 & r4 and branch to
  .Ldiffoffset_8bytes_make_align_start.
- Under .Ldiffoffset_novmx_cmp, Since r3 is unaligned we end up comparing
  "SGI" part of the string. r3's value is then aligned. r4's value is
  incremented by 3. For comparing the remaining 9 bytes, we jump to
  .Lcmp_lt32bytes.
- Here, 8 bytes of the remaining 9 bytes are compared and execution moves to
  .Lcmp_rest_lt8bytes.
- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
  value and hence ends up accessing the "next" double word. The "next" double
  word happens to occur after the last page mapped into the kernel's address
  space and hence this leads to the previously listed oops.

Thanks for the analysis.

This is just a bug, we can't read past the end of the source or dest.

How about this, works for me.

cheers

diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S
index 844d8e774492..2a302158cb53 100644
--- a/arch/powerpc/lib/memcmp_64.S
+++ b/arch/powerpc/lib/memcmp_64.S

@@ -215,20 +215,29 @@ _GLOBAL_TOC(memcmp)
 	beq	.Lzero
 
 .Lcmp_rest_lt8bytes:
-	/* Here we have only less than 8 bytes to compare with. at least s1
-	 * Address is aligned with 8 bytes.
-	 * The next double words are load and shift right with appropriate
-	 * bits.
+	/*
+	 * Here we have less than 8 bytes left to compare with. We mustn't read
+	 * past the end of either source or dest.
 	 */
-	subfic  r6,r5,8
-	slwi	r6,r6,3
-	LD	rA,0,r3
-	LD	rB,0,r4
-	srd	rA,rA,r6
-	srd	rB,rB,r6
-	cmpld	cr0,rA,rB
+
+	/* If we have less than 4 bytes, just do byte at a time */
+	cmpwi   cr1, r5, 4
+	blt	cr1, .Lshort
+
+	/* Compare 4 bytes */
+	LW	rA,0,r3
+	LW	rB,0,r4
+	cmpd	cr0,rA,rB
 	bne	cr0,.LcmpAB_lightweight
-	b	.Lzero
+
+	/* If we had exactly 4 bytes left, we're done now */
+	beq	cr1, .Lzero
+
+	/* Otherwise do what ever's left a byte at a time */
+	subi	r5, r5, 4
+	addi	r3, r3, 4
+	addi	r4, r4, 4
+	b	.Lshort
 
 .Lnon_zero:
 	mr	r3,rC

With the above patch, Linux kernel does not end up in oops. Hence,

Tested-by: Chandan Rajendra <redacted>

Thanks.

How many times had you hit the original oops? ie. was it easily
reproducible?

I could recreate the issue within 20 iterations of the test. For verifying
your patch, I let the test run for 500 iterations.

-- 
chandan

Chandan Rajendra [off-list ref] writes:

On Wednesday, February 6, 2019 5:20:04 PM IST Michael Ellerman wrote:

quoted

Chandan Rajendra [off-list ref] writes:

quoted

On Friday, February 1, 2019 4:43:52 PM IST Michael Ellerman wrote:

quoted

Michael Ellerman [off-list ref] writes:

quoted

Adding Simon who wrote the code.

Chandan Rajendra [off-list ref] writes:

quoted

When executing fstests' generic/026 test, I hit the following call trace,

[  417.061038] BUG: Unable to handle kernel data access at 0xc00000062ac40000
[  417.062172] Faulting instruction address: 0xc000000000092240
[  417.062242] Oops: Kernel access of bad area, sig: 11 [#1]
[  417.062299] LE SMP NR_CPUS=2048 DEBUG_PAGEALLOC NUMA pSeries
[  417.062366] Modules linked in:
[  417.062401] CPU: 0 PID: 27828 Comm: chacl Not tainted 5.0.0-rc2-next-20190115-00001-g6de6dba64dda #1
[  417.062495] NIP:  c000000000092240 LR: c00000000066a55c CTR: 0000000000000000
[  417.062567] REGS: c00000062c0c3430 TRAP: 0300   Not tainted  (5.0.0-rc2-next-20190115-00001-g6de6dba64dda)
[  417.062660] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000842  XER: 20000000
[  417.062750] CFAR: 00007fff7f3108ac DAR: c00000062ac40000 DSISR: 40000000 IRQMASK: 0
               GPR00: 0000000000000000 c00000062c0c36c0 c0000000017f4c00 c00000000121a660
               GPR04: c00000062ac3fff9 0000000000000004 0000000000000020 00000000275b19c4
               GPR08: 000000000000000c 46494c4500000000 5347495f41434c5f c0000000026073a0
               GPR12: 0000000000000000 c0000000027a0000 0000000000000000 0000000000000000
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
               GPR20: c00000062ea70020 c00000062c0c38d0 0000000000000002 0000000000000002
               GPR24: c00000062ac3ffe8 00000000275b19c4 0000000000000001 c00000062ac30000
               GPR28: c00000062c0c38d0 c00000062ac30050 c00000062ac30058 0000000000000000
[  417.063563] NIP [c000000000092240] memcmp+0x120/0x690
[  417.063635] LR [c00000000066a55c] xfs_attr3_leaf_lookup_int+0x53c/0x5b0
[  417.063709] Call Trace:
[  417.063744] [c00000062c0c36c0] [c00000000066a098] xfs_attr3_leaf_lookup_int+0x78/0x5b0 (unreliable)
[  417.063851] [c00000062c0c3760] [c000000000693f8c] xfs_da3_node_lookup_int+0x32c/0x5a0
[  417.063944] [c00000062c0c3820] [c0000000006634a0] xfs_attr_node_addname+0x170/0x6b0
[  417.064034] [c00000062c0c38b0] [c000000000664ffc] xfs_attr_set+0x2ac/0x340
[  417.064118] [c00000062c0c39a0] [c000000000758d40] __xfs_set_acl+0xf0/0x230
[  417.064190] [c00000062c0c3a00] [c000000000758f50] xfs_set_acl+0xd0/0x160
[  417.064268] [c00000062c0c3aa0] [c0000000004b69b0] set_posix_acl+0xc0/0x130
[  417.064339] [c00000062c0c3ae0] [c0000000004b6a88] posix_acl_xattr_set+0x68/0x110
[  417.064412] [c00000062c0c3b20] [c0000000004532d4] __vfs_setxattr+0xa4/0x110
[  417.064485] [c00000062c0c3b80] [c000000000454c2c] __vfs_setxattr_noperm+0xac/0x240
[  417.064566] [c00000062c0c3bd0] [c000000000454ee8] vfs_setxattr+0x128/0x130
[  417.064638] [c00000062c0c3c30] [c000000000455138] setxattr+0x248/0x600
[  417.064710] [c00000062c0c3d90] [c000000000455738] path_setxattr+0x108/0x120
[  417.064785] [c00000062c0c3e00] [c000000000455778] sys_setxattr+0x28/0x40
[  417.064858] [c00000062c0c3e20] [c00000000000bae4] system_call+0x5c/0x70
[  417.064930] Instruction dump:
[  417.064964] 7d201c28 7d402428 7c295040 38630008 38840008 408201f0 4200ffe8 2c050000
[  417.065051] 4182ff6c 20c50008 54c61838 7d201c28 <7d402428> 7d293436 7d4a3436 7c295040
[  417.065150] ---[ end trace 0d060411b5e3741b ]---


Both the memory locations passed to memcmp() had "SGI_ACL_FILE" and len
argument of memcmp() was set to 12. s1 argument of memcmp() had the value
0x00000000f4af0485, while s2 argument had the value 0x00000000ce9e316f.

The following is the code path within memcmp() that gets executed for the
above mentioned values,

- Since len (i.e. 12) is greater than 7, we branch to .Lno_short.
- We then prefetch the contents of r3 & r4 and branch to
  .Ldiffoffset_8bytes_make_align_start.
- Under .Ldiffoffset_novmx_cmp, Since r3 is unaligned we end up comparing
  "SGI" part of the string. r3's value is then aligned. r4's value is
  incremented by 3. For comparing the remaining 9 bytes, we jump to
  .Lcmp_lt32bytes.
- Here, 8 bytes of the remaining 9 bytes are compared and execution moves to
  .Lcmp_rest_lt8bytes.
- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
  value and hence ends up accessing the "next" double word. The "next" double
  word happens to occur after the last page mapped into the kernel's address
  space and hence this leads to the previously listed oops.

Thanks for the analysis.

This is just a bug, we can't read past the end of the source or dest.

How about this, works for me.

cheers

diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S
index 844d8e774492..2a302158cb53 100644
--- a/arch/powerpc/lib/memcmp_64.S
+++ b/arch/powerpc/lib/memcmp_64.S

@@ -215,20 +215,29 @@ _GLOBAL_TOC(memcmp)
 	beq	.Lzero
 
 .Lcmp_rest_lt8bytes:
-	/* Here we have only less than 8 bytes to compare with. at least s1
-	 * Address is aligned with 8 bytes.
-	 * The next double words are load and shift right with appropriate
-	 * bits.
+	/*
+	 * Here we have less than 8 bytes left to compare with. We mustn't read
+	 * past the end of either source or dest.
 	 */
-	subfic  r6,r5,8
-	slwi	r6,r6,3
-	LD	rA,0,r3
-	LD	rB,0,r4
-	srd	rA,rA,r6
-	srd	rB,rB,r6
-	cmpld	cr0,rA,rB
+
+	/* If we have less than 4 bytes, just do byte at a time */
+	cmpwi   cr1, r5, 4
+	blt	cr1, .Lshort
+
+	/* Compare 4 bytes */
+	LW	rA,0,r3
+	LW	rB,0,r4
+	cmpd	cr0,rA,rB
 	bne	cr0,.LcmpAB_lightweight
-	b	.Lzero
+
+	/* If we had exactly 4 bytes left, we're done now */
+	beq	cr1, .Lzero
+
+	/* Otherwise do what ever's left a byte at a time */
+	subi	r5, r5, 4
+	addi	r3, r3, 4
+	addi	r4, r4, 4
+	b	.Lshort
 
 .Lnon_zero:
 	mr	r3,rC

With the above patch, Linux kernel does not end up in oops. Hence,

Tested-by: Chandan Rajendra <redacted>

Thanks.

How many times had you hit the original oops? ie. was it easily
reproducible?

I could recreate the issue within 20 iterations of the test. For verifying
your patch, I let the test run for 500 iterations.

OK great. Thanks.

I guess I should be running fstests. What's the easiest way to do that,
is there an avocado thing for it?

cheers

On Thursday, February 7, 2019 5:27:09 AM IST Michael Ellerman wrote:

Chandan Rajendra [off-list ref] writes:

quoted

On Wednesday, February 6, 2019 5:20:04 PM IST Michael Ellerman wrote:

quoted

Chandan Rajendra [off-list ref] writes:

quoted

On Friday, February 1, 2019 4:43:52 PM IST Michael Ellerman wrote:

quoted

Michael Ellerman [off-list ref] writes:

quoted

Adding Simon who wrote the code.

Chandan Rajendra [off-list ref] writes:

quoted

When executing fstests' generic/026 test, I hit the following call trace,

[  417.061038] BUG: Unable to handle kernel data access at 0xc00000062ac40000
[  417.062172] Faulting instruction address: 0xc000000000092240
[  417.062242] Oops: Kernel access of bad area, sig: 11 [#1]
[  417.062299] LE SMP NR_CPUS=2048 DEBUG_PAGEALLOC NUMA pSeries
[  417.062366] Modules linked in:
[  417.062401] CPU: 0 PID: 27828 Comm: chacl Not tainted 5.0.0-rc2-next-20190115-00001-g6de6dba64dda #1
[  417.062495] NIP:  c000000000092240 LR: c00000000066a55c CTR: 0000000000000000
[  417.062567] REGS: c00000062c0c3430 TRAP: 0300   Not tainted  (5.0.0-rc2-next-20190115-00001-g6de6dba64dda)
[  417.062660] MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 44000842  XER: 20000000
[  417.062750] CFAR: 00007fff7f3108ac DAR: c00000062ac40000 DSISR: 40000000 IRQMASK: 0
               GPR00: 0000000000000000 c00000062c0c36c0 c0000000017f4c00 c00000000121a660
               GPR04: c00000062ac3fff9 0000000000000004 0000000000000020 00000000275b19c4
               GPR08: 000000000000000c 46494c4500000000 5347495f41434c5f c0000000026073a0
               GPR12: 0000000000000000 c0000000027a0000 0000000000000000 0000000000000000
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
               GPR20: c00000062ea70020 c00000062c0c38d0 0000000000000002 0000000000000002
               GPR24: c00000062ac3ffe8 00000000275b19c4 0000000000000001 c00000062ac30000
               GPR28: c00000062c0c38d0 c00000062ac30050 c00000062ac30058 0000000000000000
[  417.063563] NIP [c000000000092240] memcmp+0x120/0x690
[  417.063635] LR [c00000000066a55c] xfs_attr3_leaf_lookup_int+0x53c/0x5b0
[  417.063709] Call Trace:
[  417.063744] [c00000062c0c36c0] [c00000000066a098] xfs_attr3_leaf_lookup_int+0x78/0x5b0 (unreliable)
[  417.063851] [c00000062c0c3760] [c000000000693f8c] xfs_da3_node_lookup_int+0x32c/0x5a0
[  417.063944] [c00000062c0c3820] [c0000000006634a0] xfs_attr_node_addname+0x170/0x6b0
[  417.064034] [c00000062c0c38b0] [c000000000664ffc] xfs_attr_set+0x2ac/0x340
[  417.064118] [c00000062c0c39a0] [c000000000758d40] __xfs_set_acl+0xf0/0x230
[  417.064190] [c00000062c0c3a00] [c000000000758f50] xfs_set_acl+0xd0/0x160
[  417.064268] [c00000062c0c3aa0] [c0000000004b69b0] set_posix_acl+0xc0/0x130
[  417.064339] [c00000062c0c3ae0] [c0000000004b6a88] posix_acl_xattr_set+0x68/0x110
[  417.064412] [c00000062c0c3b20] [c0000000004532d4] __vfs_setxattr+0xa4/0x110
[  417.064485] [c00000062c0c3b80] [c000000000454c2c] __vfs_setxattr_noperm+0xac/0x240
[  417.064566] [c00000062c0c3bd0] [c000000000454ee8] vfs_setxattr+0x128/0x130
[  417.064638] [c00000062c0c3c30] [c000000000455138] setxattr+0x248/0x600
[  417.064710] [c00000062c0c3d90] [c000000000455738] path_setxattr+0x108/0x120
[  417.064785] [c00000062c0c3e00] [c000000000455778] sys_setxattr+0x28/0x40
[  417.064858] [c00000062c0c3e20] [c00000000000bae4] system_call+0x5c/0x70
[  417.064930] Instruction dump:
[  417.064964] 7d201c28 7d402428 7c295040 38630008 38840008 408201f0 4200ffe8 2c050000
[  417.065051] 4182ff6c 20c50008 54c61838 7d201c28 <7d402428> 7d293436 7d4a3436 7c295040
[  417.065150] ---[ end trace 0d060411b5e3741b ]---


Both the memory locations passed to memcmp() had "SGI_ACL_FILE" and len
argument of memcmp() was set to 12. s1 argument of memcmp() had the value
0x00000000f4af0485, while s2 argument had the value 0x00000000ce9e316f.

The following is the code path within memcmp() that gets executed for the
above mentioned values,

- Since len (i.e. 12) is greater than 7, we branch to .Lno_short.
- We then prefetch the contents of r3 & r4 and branch to
  .Ldiffoffset_8bytes_make_align_start.
- Under .Ldiffoffset_novmx_cmp, Since r3 is unaligned we end up comparing
  "SGI" part of the string. r3's value is then aligned. r4's value is
  incremented by 3. For comparing the remaining 9 bytes, we jump to
  .Lcmp_lt32bytes.
- Here, 8 bytes of the remaining 9 bytes are compared and execution moves to
  .Lcmp_rest_lt8bytes.
- Here we execute "LD rB,0,r4". In the case of this bug, r4 has an unaligned
  value and hence ends up accessing the "next" double word. The "next" double
  word happens to occur after the last page mapped into the kernel's address
  space and hence this leads to the previously listed oops.

Thanks for the analysis.

This is just a bug, we can't read past the end of the source or dest.

How about this, works for me.

cheers

diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S
index 844d8e774492..2a302158cb53 100644
--- a/arch/powerpc/lib/memcmp_64.S
+++ b/arch/powerpc/lib/memcmp_64.S

@@ -215,20 +215,29 @@ _GLOBAL_TOC(memcmp)
 	beq	.Lzero
 
 .Lcmp_rest_lt8bytes:
-	/* Here we have only less than 8 bytes to compare with. at least s1
-	 * Address is aligned with 8 bytes.
-	 * The next double words are load and shift right with appropriate
-	 * bits.
+	/*
+	 * Here we have less than 8 bytes left to compare with. We mustn't read
+	 * past the end of either source or dest.
 	 */
-	subfic  r6,r5,8
-	slwi	r6,r6,3
-	LD	rA,0,r3
-	LD	rB,0,r4
-	srd	rA,rA,r6
-	srd	rB,rB,r6
-	cmpld	cr0,rA,rB
+
+	/* If we have less than 4 bytes, just do byte at a time */
+	cmpwi   cr1, r5, 4
+	blt	cr1, .Lshort
+
+	/* Compare 4 bytes */
+	LW	rA,0,r3
+	LW	rB,0,r4
+	cmpd	cr0,rA,rB
 	bne	cr0,.LcmpAB_lightweight
-	b	.Lzero
+
+	/* If we had exactly 4 bytes left, we're done now */
+	beq	cr1, .Lzero
+
+	/* Otherwise do what ever's left a byte at a time */
+	subi	r5, r5, 4
+	addi	r3, r3, 4
+	addi	r4, r4, 4
+	b	.Lshort
 
 .Lnon_zero:
 	mr	r3,rC

With the above patch, Linux kernel does not end up in oops. Hence,

Tested-by: Chandan Rajendra <redacted>

Thanks.

How many times had you hit the original oops? ie. was it easily
reproducible?

I could recreate the issue within 20 iterations of the test. For verifying
your patch, I let the test run for 500 iterations.

OK great. Thanks.

I guess I should be running fstests. What's the easiest way to do that,
is there an avocado thing for it?

I just execute the main script shipped with fstests.

You can get fstests from
git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git.

Please refer to the README file for a list of devel packages that needs to be
installed before building fstests.

Build fstests by invoking "make".

You can then add the following into a file called local.config at the root
directory of fstests,
------------------------------
export TEST_DEV=/dev/loop0
export TEST_DIR=/mnt/test
export SCRATCH_DEV=/dev/loop1
export SCRATCH_MNT=/mnt/scratch
MKFS_OPTIONS="-b size=65536"
------------------------------

Create xfs filesystem on $TEST_DEV i.e. "mkfs.xfs -f -b size=65536
/dev/loop0".

Finally, invoke the test using,
# ./check generic/026

-- 
chandan