From c2da319c2e2789dccb20fdafe520ac61c9df84f7 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Sun, 13 Nov 2022 19:04:47 -0500 Subject: mm/uffd: sanity check write bit for uffd-wp protected ptes 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 Cc: Andrea Arcangeli Cc: Alistair Popple Cc: Axel Rasmussen Cc: Mike Rapoport Cc: Nadav Amit Signed-off-by: Andrew Morton --- arch/x86/include/asm/pgtable.h | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) (limited to 'arch') diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index 286a71810f9e..0564edd24ffb 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -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) -- cgit v1.2.3