[PATCH -next] powerpc/mm/ptdump: fix an undefined behaviour

Booting a power9 server with hash MMU could trigger an undefined
behaviour because pud_offset(p4d, 0) will do,

0 >> (PAGE_SHIFT:16 + PTE_INDEX_SIZE:8 + H_PMD_INDEX_SIZE:10)

 UBSAN: shift-out-of-bounds in arch/powerpc/mm/ptdump/ptdump.c:282:15
 shift exponent 34 is too large for 32-bit type 'int'
 CPU: 6 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc4-next-20200303+ #13
 Call Trace:
 dump_stack+0xf4/0x164 (unreliable)
 ubsan_epilogue+0x18/0x78
 __ubsan_handle_shift_out_of_bounds+0x160/0x21c
 walk_pagetables+0x2cc/0x700
 walk_pud at arch/powerpc/mm/ptdump/ptdump.c:282
 (inlined by) walk_pagetables at arch/powerpc/mm/ptdump/ptdump.c:311
 ptdump_check_wx+0x8c/0xf0
 mark_rodata_ro+0x48/0x80
 kernel_init+0x74/0x194
 ret_from_kernel_thread+0x5c/0x74

Fixes: 8eb07b187000 ("powerpc/mm: Dump linux pagetables")
Signed-off-by: Qian Cai <redacted>
---

Notes for maintainers:

This is on the top of the linux-next commit "powerpc: add support for
folded p4d page tables" which is in the Andrew's tree.

 arch/powerpc/mm/ptdump/ptdump.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/ptdump/ptdump.c b/arch/powerpc/mm/ptdump/ptdump.c
index 9d6256b61df3..b530f81398a7 100644
--- a/arch/powerpc/mm/ptdump/ptdump.c
+++ b/arch/powerpc/mm/ptdump/ptdump.c

@@ -279,7 +279,7 @@ static void walk_pmd(struct pg_state *st, pud_t *pud, unsigned long start)
 
 static void walk_pud(struct pg_state *st, p4d_t *p4d, unsigned long start)
 {
-	pud_t *pud = pud_offset(p4d, 0);
+	pud_t *pud = pud_offset(p4d, 0UL);
 	unsigned long addr;
 	unsigned int i;
 

-- 
2.21.0 (Apple Git-122.2)

Le 05/03/2020 à 05:47, Qian Cai a écrit :

Is that the only place we have to do this ?

(In 5.6-rc) I see the same in:
/arch/powerpc/mm/ptdump/hashpagetable.c
/arch/powerpc/kvm/book3s_64_mmu_radix.c

Wouldn't it be better to:
- Either cast addr to unsigned long in pud_index() macro
- Or change pud_index() macro to a static inline function as x86 ?

Christophe

Christophe Leroy [off-list ref] writes:

Le 05/03/2020 à 05:47, Qian Cai a écrit :

quoted

Booting a power9 server with hash MMU could trigger an undefined
behaviour because pud_offset(p4d, 0) will do,

0 >> (PAGE_SHIFT:16 + PTE_INDEX_SIZE:8 + H_PMD_INDEX_SIZE:10)

  UBSAN: shift-out-of-bounds in arch/powerpc/mm/ptdump/ptdump.c:282:15
  shift exponent 34 is too large for 32-bit type 'int'
  CPU: 6 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc4-next-20200303+ #13
  Call Trace:
  dump_stack+0xf4/0x164 (unreliable)
  ubsan_epilogue+0x18/0x78
  __ubsan_handle_shift_out_of_bounds+0x160/0x21c
  walk_pagetables+0x2cc/0x700
  walk_pud at arch/powerpc/mm/ptdump/ptdump.c:282
  (inlined by) walk_pagetables at arch/powerpc/mm/ptdump/ptdump.c:311
  ptdump_check_wx+0x8c/0xf0
  mark_rodata_ro+0x48/0x80
  kernel_init+0x74/0x194
  ret_from_kernel_thread+0x5c/0x74

Fixes: 8eb07b187000 ("powerpc/mm: Dump linux pagetables")
Signed-off-by: Qian Cai <redacted>
---

Notes for maintainers:

This is on the top of the linux-next commit "powerpc: add support for
folded p4d page tables" which is in the Andrew's tree.

  arch/powerpc/mm/ptdump/ptdump.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/ptdump/ptdump.c b/arch/powerpc/mm/ptdump/ptdump.c
index 9d6256b61df3..b530f81398a7 100644
--- a/arch/powerpc/mm/ptdump/ptdump.c
+++ b/arch/powerpc/mm/ptdump/ptdump.c

@@ -279,7 +279,7 @@ static void walk_pmd(struct pg_state *st, pud_t *pud, unsigned long start)
  
  static void walk_pud(struct pg_state *st, p4d_t *p4d, unsigned long start)
  {
-	pud_t *pud = pud_offset(p4d, 0);
+	pud_t *pud = pud_offset(p4d, 0UL);

Is that the only place we have to do this ?

(In 5.6-rc) I see the same in:
/arch/powerpc/mm/ptdump/hashpagetable.c
/arch/powerpc/kvm/book3s_64_mmu_radix.c

Wouldn't it be better to:
- Either cast addr to unsigned long in pud_index() macro
- Or change pud_index() macro to a static inline function as x86 ?

Yes, either would be better, but preferably the latter.

It's hostile to require the caller to pass an unsigned long when there's
no way they can know that's required.

cheers