On Mon, Nov 21, 2022 at 11:15:49AM +0800, Liu Shixin wrote:

On 2022/11/18 22:34, Will Deacon wrote:

quoted

On Thu, Nov 17, 2022 at 03:56:02PM +0800, Liu Shixin wrote:

quoted

The page table check trigger BUG_ON() unexpectedly when split hugepage:

 ------------[ cut here ]------------
 kernel BUG at mm/page_table_check.c:119!
 Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
 Dumping ftrace buffer:
    (ftrace buffer empty)
 Modules linked in:
 CPU: 7 PID: 210 Comm: transhuge-stres Not tainted 6.1.0-rc3+ #748
 Hardware name: linux,dummy-virt (DT)
 pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : page_table_check_set.isra.0+0x398/0x468
 lr : page_table_check_set.isra.0+0x1c0/0x468
[...]
 Call trace:
  page_table_check_set.isra.0+0x398/0x468
  __page_table_check_pte_set+0x160/0x1c0
  __split_huge_pmd_locked+0x900/0x1648
  __split_huge_pmd+0x28c/0x3b8
  unmap_page_range+0x428/0x858
  unmap_single_vma+0xf4/0x1c8
  zap_page_range+0x2b0/0x410
  madvise_vma_behavior+0xc44/0xe78
  do_madvise+0x280/0x698
  __arm64_sys_madvise+0x90/0xe8
  invoke_syscall.constprop.0+0xdc/0x1d8
  do_el0_svc+0xf4/0x3f8
  el0_svc+0x58/0x120
  el0t_64_sync_handler+0xb8/0xc0
  el0t_64_sync+0x19c/0x1a0
[...]

On arm64, pmd_leaf() will return true even if the pmd is invalid due to
pmd_present_invalid() check. So in pmdp_invalidate() the file_map_count
will not only decrease once but also increase once. Then in set_pte_at(),
the file_map_count increase again, and so trigger BUG_ON() unexpectedly.

Fix this problem by adding pmd_valid() in pmd_user_accessible_page().
Moreover, add pud_valid() for pud_user_accessible_page() too.

Fixes: 42b2547137f5 ("arm64/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
Reported-by: Denys Vlasenko <redacted>
Signed-off-by: Liu Shixin <redacted>
Acked-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 arch/arm64/include/asm/pgtable.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index edf6625ce965..3bc64199aa2e 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h

@@ -863,12 +863,12 @@ static inline bool pte_user_accessible_page(pte_t pte)
 
 static inline bool pmd_user_accessible_page(pmd_t pmd)
 {
-	return pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
+	return pmd_valid(pmd) && pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));

Hmm, doesn't this have a funny interaction with PROT_NONE where the pmd is
invalid but present? If you don't care about PROT_NONE, then you could just
do:

  pmd_valid(pmd) && !pmd_table(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd))

but if you do care then you could do:

  pmd_leaf(pmd) && !pmd_present_invalid(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd))

I prefer the latter. I will fix and resend later.

quoted

quoted

 static inline bool pud_user_accessible_page(pud_t pud)
 {
-	return pud_leaf(pud) && pud_user(pud);
+	return pud_valid(pud) && pud_leaf(pud) && pud_user(pud);

Not caused by this patch, but why don't we have something like a
pud_user_exec() check here like we do for the pte and pmd levels?

As far as I know, there is no user use the user executable pud on arm64, so didn't define pud_user_exec().

I can believe they don't get exposed to userspace at all, but exposing only
as non-executable doesn't sound right. So I would have thought that either
pud_user_accessible_page() would always return false or it would need to
check for the executable case too.

Will

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