Re: [PATCH] x86/fpu: Correct pkru/xstate inconsistency
From: Guenter Roeck
Date: Tue Feb 15 2022 - 12:02:26 EST
On Tue, Feb 15, 2022 at 8:20 AM Brian Geffon <bgeffon@xxxxxxxxxx> wrote:
>
> 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:
>
Thought so. You'll have to explain that clarly in both subject and
description to get the patch accepted to the affected stable releases.
Thanks,
Guenter
> 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(¤t->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(¤t->thread.fpu.state.xsave, XFEATURE_PKRU);
> > > + BUG_ON(!pk);
> > > + pk->pkru = pkru;
> > > __write_pkru(pkru);
> > > fpregs_unlock();
> > > }
> > > --
> > > 2.35.1.265.g69c8d7142f-goog
> > >