Re: [PATCH] x86/fpu: Correct pkru/xstate inconsistency

[Brian Geffon]

On Tue, Feb 15, 2022 at 10:57 AM Guenter Roeck <groeck@xxxxxxxxxx> wrote:
>
> Hi Brian,
>
> On Tue, Feb 15, 2022 at 7:37 AM Brian Geffon <bgeffon@xxxxxxxxxx> wrote:
> >
> > There are two issues with PKRU handling prior to 5.13. The first is that
>
> Nice catch and work. One question, though: From the above, it seems
> like this patch only applies to kernels earlier than v5.13 or, more
> specifically, to v5.4.y and v5.10.y. Is this correct, or should it be
> applied to the upstream kernel and to all applicable stable releases ?

This only applies before 5.13, so 5.10.y and 5.4.y, the behavior
decoupled PKRU from xstate in a long series from Thomas Gleixner, but
the first commit where this would have been fixed in 5.13 would be:

  commit 954436989cc550dd91aab98363240c9c0a4b7e23
  Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
  Date:   Wed Jun 23 14:02:21 2021 +0200

    x86/fpu: Remove PKRU handling from switch_fpu_finish()

>
> Thanks,
> Guenter
>
> > when eagerly switching PKRU we check that current is not a kernel
> > thread as kernel threads will never use PKRU. It's possible that
> > this_cpu_read_stable() on current_task (ie. get_current()) is returning
> > an old cached value. By forcing the read with this_cpu_read() the
> > correct task is used. Without this it's possible when switching from
> > a kernel thread to a userspace thread that we'll still observe the
> > PF_KTHREAD flag and never restore the PKRU. And as a result this
> > issue only occurs when switching from a kernel thread to a userspace
> > thread, switching from a non kernel thread works perfectly fine because
> > all we consider in that situation is the flags from some other non
> > kernel task and the next fpu is passed in to switch_fpu_finish().
> >
> > Without reloading the value finish_fpu_load() after being inlined into
> > __switch_to() uses a stale value of current:
> >
> >   ba1:   8b 35 00 00 00 00       mov    0x0(%rip),%esi
> >   ba7:   f0 41 80 4d 01 40       lock orb $0x40,0x1(%r13)
> >   bad:   e9 00 00 00 00          jmp    bb2 <__switch_to+0x1eb>
> >   bb2:   41 f6 45 3e 20          testb  $0x20,0x3e(%r13)
> >   bb7:   75 1c                   jne    bd5 <__switch_to+0x20e>
> >
> > By using this_cpu_read() and avoiding the cached value the compiler does
> > insert an additional load instruction and observes the correct value now:
> >
> >   ba1:   8b 35 00 00 00 00       mov    0x0(%rip),%esi
> >   ba7:   f0 41 80 4d 01 40       lock orb $0x40,0x1(%r13)
> >   bad:   e9 00 00 00 00          jmp    bb2 <__switch_to+0x1eb>
> >   bb2:   65 48 8b 05 00 00 00    mov    %gs:0x0(%rip),%rax
> >   bb9:   00
> >   bba:   f6 40 3e 20             testb  $0x20,0x3e(%rax)
> >   bbe:   75 1c                   jne    bdc <__switch_to+0x215>
> >
> > The second issue is when using write_pkru() we only write to the
> > xstate when the feature bit is set because get_xsave_addr() returns
> > NULL when the feature bit is not set. This is problematic as the CPU
> > is free to clear the feature bit when it observes the xstate in the
> > init state, this behavior seems to be documented a few places throughout
> > the kernel. If the bit was cleared then in write_pkru() we would happily
> > write to PKRU without ever updating the xstate, and the FPU restore on
> > return to userspace would load the old value agian.
> >
> > Fixes: 0cecca9d03c9 ("x86/fpu: Eager switch PKRU state")
> > Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx>
> > Signed-off-by: Willis Kung <williskung@xxxxxxxxxx>
> > Tested-by: Willis Kung <williskung@xxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/fpu/internal.h |  2 +-
> >  arch/x86/include/asm/pgtable.h      | 14 ++++++++++----
> >  2 files changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> > index 03b3de491b5e..540bda5bdd28 100644
> > --- a/arch/x86/include/asm/fpu/internal.h
> > +++ b/arch/x86/include/asm/fpu/internal.h
> > @@ -598,7 +598,7 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
> >          * PKRU state is switched eagerly because it needs to be valid before we
> >          * return to userland e.g. for a copy_to_user() operation.
> >          */
> > -       if (!(current->flags & PF_KTHREAD)) {
> > +       if (!(this_cpu_read(current_task)->flags & PF_KTHREAD)) {
> >                 /*
> >                  * If the PKRU bit in xsave.header.xfeatures is not set,
> >                  * then the PKRU component was in init state, which means
> > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> > index 9e71bf86d8d0..aa381b530de0 100644
> > --- a/arch/x86/include/asm/pgtable.h
> > +++ b/arch/x86/include/asm/pgtable.h
> > @@ -140,16 +140,22 @@ static inline void write_pkru(u32 pkru)
> >         if (!boot_cpu_has(X86_FEATURE_OSPKE))
> >                 return;
> >
> > -       pk = get_xsave_addr(&current->thread.fpu.state.xsave, XFEATURE_PKRU);
> > -
> >         /*
> >          * The PKRU value in xstate needs to be in sync with the value that is
> >          * written to the CPU. The FPU restore on return to userland would
> >          * otherwise load the previous value again.
> >          */
> >         fpregs_lock();
> > -       if (pk)
> > -               pk->pkru = pkru;
> > +       /*
> > +        * The CPU is free to clear the feature bit when the xstate is in the
> > +        * init state. For this reason, we need to make sure the feature bit is
> > +        * reset when we're explicitly writing to pkru. If we did not then we
> > +        * would write to pkru and it would not be saved on a context switch.
> > +        */
> > +       current->thread.fpu.state.xsave.header.xfeatures |= XFEATURE_MASK_PKRU;
> > +       pk = get_xsave_addr(&current->thread.fpu.state.xsave, XFEATURE_PKRU);
> > +       BUG_ON(!pk);
> > +       pk->pkru = pkru;
> >         __write_pkru(pkru);
> >         fpregs_unlock();
> >  }
> > --
> > 2.35.1.265.g69c8d7142f-goog
> >