Re: [kvm-unit-tests PATCH] x86: Look up the PTE rather than assuming it

[Jim Mattson <jmattson@google.com>]

On Fri, Oct 29, 2021 at 3:48 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Oct 29, 2021, Aaron Lewis wrote:
> > Rather than assuming which PTE the SMEP test is running on, look it up
> > to ensure we have the correct entry.  If this test were to run on a
> > different page table (ie: run in an L2 test) the wrong PTE would be set.
> > Switch to looking up the PTE to avoid this from happening.
> >
> > Signed-off-by: Aaron Lewis <aaronlewis@google.com>
> > ---
> >  x86/access.c   | 9 ++++++---
> >  x86/cstart64.S | 1 -
> >  2 files changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/x86/access.c b/x86/access.c
> > index 4725bbd..a4d72d9 100644
> > --- a/x86/access.c
> > +++ b/x86/access.c
> > @@ -204,7 +204,7 @@ static void set_cr0_wp(int wp)
> >  static unsigned set_cr4_smep(int smep)
> >  {
> >      unsigned long cr4 = shadow_cr4;
> > -    extern u64 ptl2[];
> > +    pteval_t *pte;
> >      unsigned r;
> >
> >      cr4 &= ~CR4_SMEP_MASK;
> > @@ -213,11 +213,14 @@ static unsigned set_cr4_smep(int smep)
> >      if (cr4 == shadow_cr4)
> >          return 0;
> >
> > +    pte = get_pte(phys_to_virt(read_cr3()), set_cr4_smep);
>
> What guarantees are there that set_cr4_smep() and the rest of the test are mapped
> by the same PTE?  I 100% agree the current code is ugly, e.g. I can't remember how
> ptl2[2] is guaranteed to be used, but if we're going to fix it then we should aim
> for a more robust solution.

One possible solution is to put labels around the code that has to run
with SMEP and then to implement a function to modify the PTEs for a
range of addresses, using gcc's double-ampersand syntax to get the
addresses of the labels.

> > +    assert(pte);
> > +
> >      if (smep)
> > -        ptl2[2] &= ~PT_USER_MASK;
> > +        *pte &= ~PT_USER_MASK;
> >      r = write_cr4_checking(cr4);
> >      if (r || !smep) {
> > -        ptl2[2] |= PT_USER_MASK;
> > +        *pte |= PT_USER_MASK;
> >
> >       /* Flush to avoid spurious #PF */
> >       invlpg((void *)(2 << 21));
>
> This invlpg() should be updated as well.
>
> > diff --git a/x86/cstart64.S b/x86/cstart64.S
> > index 5c6ad38..4ba9943 100644
> > --- a/x86/cstart64.S
> > +++ b/x86/cstart64.S
> > @@ -26,7 +26,6 @@ ring0stacktop:
> >  .data
> >
> >  .align 4096
> > -.globl ptl2
> >  ptl2:
> >  i = 0
> >       .rept 512 * 4
> > --
> > 2.33.1.1089.g2158813163f-goog
> >