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

[Juergen Gross <jgross@suse.com>]

[-- Attachment #1.1.1: Type: text/plain, Size: 1937 bytes --]

On 16.07.26 14:35, David Laight wrote:
> On Thu, 16 Jul 2026 12:18:48 +0300
> Mike Rapoport <rppt@kernel.org> wrote:
> 
>> On Thu, Jul 16, 2026 at 10:37:00AM +0200, Jürgen Groß wrote:
>>> On 16.07.26 10:10, 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 bit 1 while *rw only ever holds 0 or 1, so the AND is
>>>> always 0.  *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.
>>>>
>>>> Accumulate NX and RW in unsigned long locals in their native bit positions
>>>> and store the result into the bool outputs once.
>>>>
>>>> Fixes: ceb647b4b529 ("x86/pat: Introduce lookup_address_in_pgd_attr()")
>>>> Assisted-by: Copilot:claude-opus-4.8
>>>> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>>>
>>> Thanks for catching this.
>>>
>>> Reviewed-by: Juergen Gross <jgross@suse.com>
>>>
>>> Just one remark: instead of using additional local variables the fix could
>>> just look like:
>>>
>>>      *rw |= !!(pXd_flags(*pXd) & _PAGE_RW);
>>>
>>> I don't really care how the issue is fixed, but this would result in less
>>> code.
>>
>> But it will be slower :)
>>
>> I instrumented cpa-test and I see ~2% improvement with additional local
>> variables.
> 
> Does it improve further if you defer the '& _PAGE_NX' to the final assigment?

If so, it would probably be beneficial to have one local variable for the
logical OR of all page table entries involved (used for *nx), and one for
the logical AND of all entries (used for *rw).


Juergen

[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3743 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]