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

[Mike Rapoport]

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@xxxxxxxxxx>
> 
> Thanks for catching this.
> 
> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
> 
> 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.
 
> Juergen






-- 
Sincerely yours,
Mike.