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Wed, 22 Oct 2025 15:08:14 -0700 (PDT) From: Usama Arif To: dwmw@amazon.co.uk, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, ardb@kernel.org, hpa@zytor.com Cc: x86@kernel.org, apopple@nvidia.com, thuth@redhat.com, nik.borisov@suse.com, kas@kernel.org, linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org, kernel-team@meta.com, Usama Arif , Michael van der Westhuizen , Tobias Fleig Subject: [PATCH 3/3] x86/mm: Move _PAGE_BIT_NOPTISHADOW from bit 58 to bit 9 Date: Wed, 22 Oct 2025 23:06:27 +0100 Message-ID: <20251022220755.1026144-4-usamaarif642@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20251022220755.1026144-1-usamaarif642@gmail.com> References: <20251022220755.1026144-1-usamaarif642@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Kexec from a kernel with 5-level page tables to one with 4-level page tables is broken because bits above the physical address width are not properly masked by the target kernel. This issue was particularly triggered by _PAGE_BIT_NOPTISHADOW, which uses _PAGE_BIT_SOFTW5 (bit 58). The ideal fix would be to mask the upper bits properly in all kernels. However, this is not feasible due to: - The logistical challenge of patching all older kernels in production - The patch not being applicable for live patching Instead, move _PAGE_BIT_NOPTISHADOW to use _PAGE_BIT_SOFTW1 (bit 9), which is already masked by older kernels using PAGE_MASK. This is safe as the other users of _PAGE_BIT_SOFTW1 (_PAGE_BIT_SPECIAL and _PAGE_BIT_CPA_TEST) are only used for leaf entries, while _PAGE_BIT_NOPTISHADOW is used for PGD and P4D entries only. Fixes: d0ceea662d45 ("x86/mm: Add _PAGE_NOPTISHADOW bit to avoid updating u= serspace page tables") Co-developed-by: Kiryl Shutsemau Signed-off-by: Kiryl Shutsemau Signed-off-by: Usama Arif Reported-by: Michael van der Westhuizen Reported-by: Tobias Fleig --- arch/x86/include/asm/pgtable_types.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pg= table_types.h index 2ec250ba467e2..616e928d87973 100644 --- a/arch/x86/include/asm/pgtable_types.h +++ b/arch/x86/include/asm/pgtable_types.h @@ -29,6 +29,8 @@ #define _PAGE_BIT_PKEY_BIT3 62 /* Protection Keys, bit 4/4 */ #define _PAGE_BIT_NX 63 /* No execute: only valid after cpuid check */ =20 +/* _PAGE_BIT_SPECIAL and _PAGE_BIT_CPA_TEST only used for leaf entries */ +#define _PAGE_BIT_NOPTISHADOW _PAGE_BIT_SOFTW1 #define _PAGE_BIT_SPECIAL _PAGE_BIT_SOFTW1 #define _PAGE_BIT_CPA_TEST _PAGE_BIT_SOFTW1 #define _PAGE_BIT_UFFD_WP _PAGE_BIT_SOFTW2 /* userfaultfd wrprotected */ @@ -37,11 +39,9 @@ =20 #ifdef CONFIG_X86_64 #define _PAGE_BIT_SAVED_DIRTY _PAGE_BIT_SOFTW5 /* Saved Dirty bit (leaf) */ -#define _PAGE_BIT_NOPTISHADOW _PAGE_BIT_SOFTW5 /* No PTI shadow (root PGD)= */ #else /* Shared with _PAGE_BIT_UFFD_WP which is not supported on 32 bit */ #define _PAGE_BIT_SAVED_DIRTY _PAGE_BIT_SOFTW2 /* Saved Dirty bit (leaf) */ -#define _PAGE_BIT_NOPTISHADOW _PAGE_BIT_SOFTW2 /* No PTI shadow (root PGD)= */ #endif =20 /* If _PAGE_BIT_PRESENT is clear, we use these: */ --=20 2.47.3