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mm/uffd: sanity check write bit for uffd-wp protected ptes

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Let's add one sanity check for CONFIG_DEBUG_VM on the write bit in
whatever chance we have when walking through the pgtables.  It can bring
the error earlier even before the app notices the data was corrupted on
the snapshot.  Also it helps us to identify this is a wrong pgtable setup,
so hopefully a great information to have for debugging too.

Link: https://lkml.kernel.org/r/20221114000447.1681003-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Peter Xu 2022-11-13 19:04:47 -05:00 committed by Andrew Morton
parent 25e9fa22fb
commit c2da319c2e
1 changed files with 17 additions and 1 deletions
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18
arch/x86/include/asm/pgtable.h
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@ -292,7 +292,23 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
static inline int pte_uffd_wp(pte_t pte)
{
return pte_flags(pte) & _PAGE_UFFD_WP;
bool wp = pte_flags(pte) & _PAGE_UFFD_WP;
#ifdef CONFIG_DEBUG_VM
/*
* Having write bit for wr-protect-marked present ptes is fatal,
* because it means the uffd-wp bit will be ignored and write will
* just go through.
*
* Use any chance of pgtable walking to verify this (e.g., when
* page swapped out or being migrated for all purposes). It means
* something is already wrong. Tell the admin even before the
* process crashes. We also nail it with wrong pgtable setup.
*/
WARN_ON_ONCE(wp && pte_write(pte));
#endif
return wp;
}
static inline pte_t pte_mkuffd_wp(pte_t pte)