linux-kernel - Re: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore

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Date:   Fri, 20 Dec 2019 12:32:36 -0800
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when
 __fpu_restore_sig() fails

On Fri, 2019-12-20 at 21:16 +0100, Sebastian Andrzej Siewior wrote:
> [...]
> Now that I looked at it:
> All kernel loads don't fail. If they fail we end up in the handler and
> restore to init-state. So no need to reset `fpu_fpregs_owner_ctx' in this
> case. The variable is actually set to task's FPU state so resetting is
> not required.

Agree.

> fpu__save() invokes copy_kernel_to_fpregs() (on older boxes) and by
> resetting `fpu_fpregs_owner_ctx' we would load it twice (in fpu__save()
> and on return to userland).

That is true.

> So far I can tell, the only problematic case is the signal code because
> here the state restore *may* fail and we *may* do it in two steps. The
> error happens only if both `may' are true.
> 
> > > So if this patch works for you and you don't find anything else where it
> > > falls apart then I will audit tomorrow all callers which got the
> > > "invalidator" added and check for that angle.
> > 
> > Yes, that works for me.  Also, most of these call sites are under fpregs_lock(),
> > and we could use __cpu_invalidate_fpregs_state().
> > I was also thinking maybe add warnings when any new code re-introduces the issue,
> > but not sure where to add that.  Do you think that is needed?
> 
> I was thinking about it. So the `read-FPU-state' function must be
> invoked within the fpregs_lock() section. This could be easily
> enforced. At fpregs_unlock() time `fpu_fpregs_owner_ctx' must be NULL or
> pointing to task's FPU.
> My brain is fried for today so I'm sure if this is a sane approach. But
> it might be a start.

I will also think about it.  Thanks!

Yu-cheng