In case of a use after free kernel OOPs, freed path of the object is
required to debug futher. In most of cases the object address is present
in one of the registers.
Thus check the register's address and if it belongs to slab, print its
alloc and free path.
commit a02a25709155 ("mm/slub: add support for free path information of an object")
provides free path along with alloc path of object in mem_dump_obj().
Thus call it with register values same as in ARM with
commit 14c0508adcdb ("arm: print alloc free paths for address in registers")
e.g. in the below issue register x20 belongs to slab, and a use after free
issue occurred on one of its dereferenced values:
[ 19.516507] Unable to handle kernel paging request at virtual address 006b6b6b6b6b6b73
..
..
[ 19.528784] Register x10 information: 0-page vmalloc region starting at 0xffff800011bb0000 allocated at paging_init+0x1d8/0x544
[ 19.529143] Register x11 information: 0-page vmalloc region starting at 0xffff800011bb0000 allocated at paging_init+0x1d8/0x544
[ 19.529513] Register x12 information: non-paged memory
..
[ 19.544953] Register x20 information: slab kmalloc-128 start ffff0000c3a34280 data offset 128 pointer offset 0 size 128 allocated at meminfo_proc_show+0x44/0x588
[ 19.545432] ___slab_alloc+0x638/0x658
[ 19.545576] __slab_alloc.isra.0+0x2c/0x58
[ 19.545728] kmem_cache_alloc+0x584/0x598
[ 19.545877] meminfo_proc_show+0x44/0x588
[ 19.546022] seq_read_iter+0x258/0x460
[ 19.546160] proc_reg_read_iter+0x90/0xd0
[ 19.546308] generic_file_splice_read+0xd0/0x188
[ 19.546474] do_splice_to+0x90/0xe0
[ 19.546609] splice_direct_to_actor+0xbc/0x240
[ 19.546768] do_splice_direct+0x8c/0xe8
[ 19.546911] do_sendfile+0x2c4/0x500
[ 19.547048] __arm64_sys_sendfile64+0x160/0x168
[ 19.547205] el0_svc_common.constprop.0+0x60/0x120
[ 19.547377] do_el0_svc_compat+0x1c/0x40
[ 19.547524] el0_svc_compat+0x24/0x38
[ 19.547660] el0_sync_compat_handler+0x90/0x158
[ 19.547821] Free path:
[ 19.547906] __slab_free+0x3dc/0x538
[ 19.548051] kfree+0x2d8/0x310
[ 19.548176] meminfo_proc_show+0x60/0x588
[ 19.548322] seq_read_iter+0x258/0x460
[ 19.548459] proc_reg_read_iter+0x90/0xd0
[ 19.548602] generic_file_splice_read+0xd0/0x188
[ 19.548761] do_splice_to+0x90/0xe0
[ 19.548889] splice_direct_to_actor+0xbc/0x240
[ 19.549040] do_splice_direct+0x8c/0xe8
[ 19.549183] do_sendfile+0x2c4/0x500
[ 19.549319] __arm64_sys_sendfile64+0x160/0x168
[ 19.549477] el0_svc_common.constprop.0+0x60/0x120
[ 19.549646] do_el0_svc_compat+0x1c/0x40
[ 19.549782] el0_svc_compat+0x24/0x38
[ 19.549913] el0_sync_compat_handler+0x90/0x158
[ 19.550067] el0_sync_compat+0x174/0x180
..
Signed-off-by: Vaneet Narang <redacted>
Signed-off-by: Maninder Singh <redacted>
---
arch/arm64/include/asm/system_misc.h | 1 +
arch/arm64/kernel/process.c | 11 +++++++++++
arch/arm64/kernel/traps.c | 1 +
3 files changed, 13 insertions(+)
From: Mark Rutland <mark.rutland@arm.com> Date: 2021-03-24 11:52:51
Hi,
On Wed, Mar 24, 2021 at 12:24:59PM +0530, Maninder Singh wrote:
In case of a use after free kernel OOPs, freed path of the object is
required to debug futher. In most of cases the object address is present
in one of the registers.
Thus check the register's address and if it belongs to slab, print its
alloc and free path.
This path is used for a number of failures that might have nothing to do
with a use-after-free, and from the trimmed example below it looks like
this could significantly bloat the panic and potentially cause important
information to be lost from the log, especially given the large number
of GPRs arm64 has.
Given that, I suspect this is not something we want enabled by default.
When is this logging enabled? I assume the kernel doesn't always record
the alloc/free paths. Is there a boot-time option to control this?
How many lines does this produce on average?
quoted hunk
commit a02a25709155 ("mm/slub: add support for free path information of an object")
provides free path along with alloc path of object in mem_dump_obj().
Thus call it with register values same as in ARM with
commit 14c0508adcdb ("arm: print alloc free paths for address in registers")
e.g. in the below issue register x20 belongs to slab, and a use after free
issue occurred on one of its dereferenced values:
[ 19.516507] Unable to handle kernel paging request at virtual address 006b6b6b6b6b6b73
..
..
[ 19.528784] Register x10 information: 0-page vmalloc region starting at 0xffff800011bb0000 allocated at paging_init+0x1d8/0x544
[ 19.529143] Register x11 information: 0-page vmalloc region starting at 0xffff800011bb0000 allocated at paging_init+0x1d8/0x544
[ 19.529513] Register x12 information: non-paged memory
..
[ 19.544953] Register x20 information: slab kmalloc-128 start ffff0000c3a34280 data offset 128 pointer offset 0 size 128 allocated at meminfo_proc_show+0x44/0x588
[ 19.545432] ___slab_alloc+0x638/0x658
[ 19.545576] __slab_alloc.isra.0+0x2c/0x58
[ 19.545728] kmem_cache_alloc+0x584/0x598
[ 19.545877] meminfo_proc_show+0x44/0x588
[ 19.546022] seq_read_iter+0x258/0x460
[ 19.546160] proc_reg_read_iter+0x90/0xd0
[ 19.546308] generic_file_splice_read+0xd0/0x188
[ 19.546474] do_splice_to+0x90/0xe0
[ 19.546609] splice_direct_to_actor+0xbc/0x240
[ 19.546768] do_splice_direct+0x8c/0xe8
[ 19.546911] do_sendfile+0x2c4/0x500
[ 19.547048] __arm64_sys_sendfile64+0x160/0x168
[ 19.547205] el0_svc_common.constprop.0+0x60/0x120
[ 19.547377] do_el0_svc_compat+0x1c/0x40
[ 19.547524] el0_svc_compat+0x24/0x38
[ 19.547660] el0_sync_compat_handler+0x90/0x158
[ 19.547821] Free path:
[ 19.547906] __slab_free+0x3dc/0x538
[ 19.548051] kfree+0x2d8/0x310
[ 19.548176] meminfo_proc_show+0x60/0x588
[ 19.548322] seq_read_iter+0x258/0x460
[ 19.548459] proc_reg_read_iter+0x90/0xd0
[ 19.548602] generic_file_splice_read+0xd0/0x188
[ 19.548761] do_splice_to+0x90/0xe0
[ 19.548889] splice_direct_to_actor+0xbc/0x240
[ 19.549040] do_splice_direct+0x8c/0xe8
[ 19.549183] do_sendfile+0x2c4/0x500
[ 19.549319] __arm64_sys_sendfile64+0x160/0x168
[ 19.549477] el0_svc_common.constprop.0+0x60/0x120
[ 19.549646] do_el0_svc_compat+0x1c/0x40
[ 19.549782] el0_svc_compat+0x24/0x38
[ 19.549913] el0_sync_compat_handler+0x90/0x158
[ 19.550067] el0_sync_compat+0x174/0x180
..
Signed-off-by: Vaneet Narang <redacted>
Signed-off-by: Maninder Singh <redacted>
---
arch/arm64/include/asm/system_misc.h | 1 +
arch/arm64/kernel/process.c | 11 +++++++++++
arch/arm64/kernel/traps.c | 1 +
3 files changed, 13 insertions(+)
@@ -306,6 +306,17 @@ void __show_regs(struct pt_regs *regs)}}+void__show_regs_alloc_free(structpt_regs*regs)+{+inti;++/* check for x0 - x29 only */
Why x29? The AAPCS says that's the frame pointer, so much like the SP it
shouldn't point to a heap object.
+ for (i = 0; i <= 29; i++) {
+ pr_alert("Register x%d information:", i);
+ mem_dump_obj((void *)regs->regs[i]);
+ }
+}
The pr_alert() is unconditional -- can mem_dumpo_obj() never be
disabled?
What loglevel does mem_dump_obj() use? Generally we try to keep that
matched, so I'm surprised it isn't taken as a parameter.
@@ -104,6 +104,7 @@ static int __die(const char *str, int err, struct pt_regs *regs)print_modules();show_regs(regs);+__show_regs_alloc_free(regs);
As above, I'm not sure this is the right place to put this. We can get
here for reasons other than UAF, and I'm sure we can trigger panics via
UAF without going via this.
THanks,
Mark.
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linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
This path is used for a number of failures that might have nothing to do
with a use-after-free, and from the trimmed example below it looks like
this could significantly bloat the panic and potentially cause important
information to be lost from the log, especially given the large number
of GPRs arm64 has.
Given that, I suspect this is not something we want enabled by default.
Yes it will add a lot of logs in case of normal die also.
But it can suggest free and alloc paths which can help in some debugging.
When is this logging enabled? I assume the kernel doesn't always record
the alloc/free paths. Is there a boot-time option to control this?
if SLUB_DEBUG_ON is enabled at build time then it is enabled from boot,
otherwise it can be enabled by kernel command line parameter of "slub_debug=u"
if SLUB_DEBUG is enabled.
How many lines does this produce on average?
16 traces at max for alloc and 16 for free path.
so in total for slab object 34 lines will be printed,
otherwise one line for each object info of vmalloc.
quoted
+void __show_regs_alloc_free(struct pt_regs *regs)
+{
+ int i;
+
+ /* check for x0 - x29 only */
Why x29? The AAPCS says that's the frame pointer, so much like the SP it
shouldn't point to a heap object.
yes x29 can be ignored.
quoted
+ for (i = 0; i <= 29; i++) {
+ pr_alert("Register x%d information:", i);
+ mem_dump_obj((void *)regs->regs[i]);
+ }
+}
The pr_alert() is unconditional -- can mem_dumpo_obj() never be
disabled?
mem_dump_obj is dependent on CONFIG_PRINTK
What loglevel does mem_dump_obj() use? Generally we try to keep that
matched, so I'm surprised it isn't taken as a parameter.
loglevel of mem_dump_obj is pr_cont, so it will be with the caller's loglevel
/**
* mem_dump_obj - Print available provenance information
* @object: object for which to find provenance information.
*
* This function uses pr_cont(), so that the caller is expected to have
* printed out whatever preamble is appropriate.
But loglvel will be changed to pr_info when it goes for printing of traces.
void kmem_dump_obj(void *object)
{
....
pr_cont("\n");
for (i = 0; i < ARRAY_SIZE(kp.kp_stack); i++) {
if (!kp.kp_stack[i])
break;
pr_info(" %pS\n", kp.kp_stack[i]);
}
...
@@ -104,6 +104,7 @@ static int __die(const char *str, int err, struct pt_regs *regs)print_modules();show_regs(regs);+__show_regs_alloc_free(regs);
As above, I'm not sure this is the right place to put this. We can get
here for reasons other than UAF, and I'm sure we can trigger panics via
UAF without going via this.
Adding call here, because we though in case of use after free __die will be called.
due to unhandled page fault of 0x6b6b6 MAGIC value. thats why picked this place.
Thanks,
Maninder Singh
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