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

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Date:   Thu, 19 Dec 2019 08:44:08 -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 Thu, 2019-12-19 at 15:22 +0100, Sebastian Andrzej Siewior wrote:
> On 2019-12-18 12:53:59 [-0800], Yu-cheng Yu wrote:
> > I could have explained this better, sorry!  I will explain the first
> > case below; other cases are similar.
> > 
> > In copy_user_to_fpregs_zeroing(), we have:
> > 
> >     if (user_xsave()) {
> >         ...
> >         if (unlikely(init_bv))
> >             copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
> >         return copy_user_to_xregs(buf, xbv);
> >         ...
> >     }
> > 
> > The copy_user_to_xregs() may fail, and when that happens, before going to
> > the slow path, there is fpregs_unlock() and context switches may happen.
> 
> The context switch may only happen after fpregs_unlock().
> 
> > However, at this point, fpu_fpregs_owner_ctx has not been changed; it could
> > still be another task's FPU.
> 
> TIF_NEED_FPU_LOAD is set for the task in __fpu__restore_sig() and its
> context (__fpu_invalidate_fpregs_state()) has been invalidated. So the
> FPU register may contain another task's content and
> fpu_fpregs_owner_ctx points to another context.
> 
> >                               For this to happen and to be detected, the user
> > stack page needs to be non-present, fpu_fpregs_owner_ctx need to be another task,
> > and that other task needs to be able to detect its registers are modified.
> > The last factor is not easy to reproduce, and a CET control-protection fault
> > helps.
> 
> So far everything is legal. However. If there is a context switch before
> fpregs_lock() then this is bad before we don't account for that.
> So that:
> 
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -352,6 +352,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  			fpregs_unlock();
>  			return 0;
>  		}
> +		fpregs_deactivate(fpu);
>  		fpregs_unlock();
>  	}
>  
> @@ -403,6 +404,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  	}
>  	if (!ret)
>  		fpregs_mark_activate();
> +	else
> +		fpregs_deactivate(fpu);
>  	fpregs_unlock();
>  
>  err_out:
> 
> 
> Should be enough.

Yes, this works.  But then everywhere that calls copy_*_to_xregs_*() etc. needs to be checked.
Are there other alternatives?

Yu-cheng