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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id cb5-20020a05622a1f8500b0039cc0fbdb61sm4870380qtb.53.2022.11.13.16.04.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Nov 2022 16:04:52 -0800 (PST) From: Peter Xu To: linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: Mike Rapoport , Nadav Amit , Andrew Morton , peterx@redhat.com, Andrea Arcangeli , Ives van Hoorne , Axel Rasmussen , Alistair Popple Subject: [PATCH v3 2/2] mm/uffd: Sanity check write bit for uffd-wp protected ptes Date: Sun, 13 Nov 2022 19:04:47 -0500 Message-Id: <20221114000447.1681003-3-peterx@redhat.com> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20221114000447.1681003-1-peterx@redhat.com> References: <20221114000447.1681003-1-peterx@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" 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. Cc: Andrea Arcangeli Signed-off-by: Peter Xu --- arch/x86/include/asm/pgtable.h | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index 5059799bebe3..63bdbb0f989e 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -291,7 +291,23 @@ 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 =3D 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; } =20 static inline pte_t pte_mkuffd_wp(pte_t pte) --=20 2.37.3