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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49548e14e1bsm30017445e9.2.2026.07.17.03.32.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 17 Jul 2026 03:32:16 -0700 (PDT) Date: Fri, 17 Jul 2026 11:32:09 +0100 From: David Laight To: "Mike Rapoport (Microsoft)" Cc: Dave Hansen , Andy Lutomirski , Borislav Petkov , Ingo Molnar , Ingo Molnar , Juergen Gross , "H. Peter Anvin" , Peter Zijlstra , Thomas Gleixner , x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] x86/mm/pat: fix effective RW computation in lookup_address_in_pgd_attr() Message-ID: <20260717113209.178573db@pumpkin> In-Reply-To: <20260717-verify-rwx-fix-v2-1-fc23141311d6@kernel.org> References: <20260717-verify-rwx-fix-v2-1-fc23141311d6@kernel.org> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 17 Jul 2026 12:41:43 +0300 "Mike Rapoport (Microsoft)" wrote: > lookup_address_in_pgd_attr() accumulates the effective NX and RW bits of > the walked page table levels so that verify_rwx() can detect mappings that > are both writable and executable. > > The RW bits are folded into a bool with > > rw &= pXd_flags(*pXd) & _PAGE_RW; > > but _PAGE_RW is 0x2. So consider the accumulation line: > > rw &= pXd_flags(*pXd) & _PAGE_RW; > > where rw=0x1 and the right side evaluates down to 0x2. It'll end up doing: > > rw = 0x1 & 0x2 > > and rw always ends up 0. > > This way rw becomes false at the first level walked, regardless of the > actual permissions, and verify_rwx() treats every mapping as non-writable > and never reports a W^X violation. > > Add double negation to the right side to normalize the _PAGE_RW flag to > 0 or 1. > > Fixes: ceb647b4b529 ("x86/pat: Introduce lookup_address_in_pgd_attr()") > Assisted-by: Copilot:claude-opus-4.8 > Signed-off-by: Mike Rapoport (Microsoft) > Reviewed-by: Juergen Gross > --- > v2 changes: > * use a simpler version that uses !! to ensure the comparison result is > 0 or 1 Isn't this slower (and probably larger) because of the repeated references to the parameter passed by reference? David > > v1: https://patch.msgid.link/20260716-verify-rwx-fix-v1-1-d8a04854df06@kernel.org > --- > arch/x86/mm/pat/set_memory.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c > index d023a40a1e03..cd9ecc8e52f9 100644 > --- a/arch/x86/mm/pat/set_memory.c > +++ b/arch/x86/mm/pat/set_memory.c > @@ -731,7 +731,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address, > > *level = PG_LEVEL_512G; > *nx |= pgd_flags(*pgd) & _PAGE_NX; > - *rw &= pgd_flags(*pgd) & _PAGE_RW; > + *rw &= !!(pgd_flags(*pgd) & _PAGE_RW); > > p4d = p4d_offset(pgd, address); > if (p4d_none(*p4d)) > @@ -742,7 +742,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address, > > *level = PG_LEVEL_1G; > *nx |= p4d_flags(*p4d) & _PAGE_NX; > - *rw &= p4d_flags(*p4d) & _PAGE_RW; > + *rw &= !!(p4d_flags(*p4d) & _PAGE_RW); > > pud = pud_offset(p4d, address); > if (pud_none(*pud)) > @@ -753,7 +753,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address, > > *level = PG_LEVEL_2M; > *nx |= pud_flags(*pud) & _PAGE_NX; > - *rw &= pud_flags(*pud) & _PAGE_RW; > + *rw &= !!(pud_flags(*pud) & _PAGE_RW); > > pmd = pmd_offset(pud, address); > if (pmd_none(*pmd)) > @@ -764,7 +764,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address, > > *level = PG_LEVEL_4K; > *nx |= pmd_flags(*pmd) & _PAGE_NX; > - *rw &= pmd_flags(*pmd) & _PAGE_RW; > + *rw &= !!(pmd_flags(*pmd) & _PAGE_RW); > > return pte_offset_kernel(pmd, address); > } > > --- > base-commit: a13c140cc289c0b7b3770bce5b3ad42ab35074aa > change-id: 20260716-verify-rwx-fix-3b0bd3e51244 > > -- > Sincerely yours, > Mike. >