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

[Jürgen Groß]

On 16.07.26 14:49, David Laight wrote:
> On Thu, 16 Jul 2026 14:46:48 +0200
> Juergen Gross <jgross@xxxxxxxx> wrote:
>
> > On 16.07.26 14:35, David Laight wrote:
> > > On Thu, 16 Jul 2026 12:18:48 +0300
> > > Mike Rapoport <rppt@xxxxxxxxxx> 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@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.
> > >
> > > 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).
>
> Which is what the patch does...

Yes, of course, but you could delay masking the RW and NX bits until the final
assignment to *ret_nx and *ret_rw. At the same time I'd fetch the flags only
once per level. So something like:

unsigned long flags_ored = 0;
unsigned long flags_anded = ~0UL;
unsigned long flags;

flags = pgd_flags(*pgd)
flags_ored |= flags;
flags_anded &= flags;

...

*ret_nx = !!(flags_ored & _PAGE_NX);
*ret_rw = !!(flags_anded & _PAGE_RW);


Juergen

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