Re: [PATCH v2 5/5] x86/mm/pat: fix effective RW computation in lookup_address_in_pgd_attr()

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Thu, Aug 13, 2026 at 12:01:28PM +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()")
> Cc: stable@vger.kernel.org
> Assisted-by: Copilot:claude-opus-4.8
> Reviewed-by: Juergen Gross <jgross@suse.com>
> Tested-by: syzbot@syzkaller.appspotmail.com
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Yikes, good find! LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@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 fbc418dfc597..430d0b448371 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -754,7 +754,7 @@ 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))
> @@ -765,7 +765,7 @@ 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))
> @@ -776,7 +776,7 @@ 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))
> @@ -787,7 +787,7 @@ 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);
>  }
>
> --
> 2.53.0
>

--
Cheers, Lorenzo