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From: Yu-cheng Yu <yu-cheng.yu@intel.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Rik van Riel <riel@surriel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails
Date: Thu, 19 Dec 2019 08:44:08 -0800
Message-ID: <19a94f88f1bc66bb81dbf5dd72083d03ca5090e9.camel@intel.com> (raw)
In-Reply-To: <20191219142217.axgpqlb7zzluoxnf@linutronix.de>

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


  reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-12 21:08 [PATCH v2 0/3] Fix small issues in XSAVES Yu-cheng Yu
2019-12-12 21:08 ` [PATCH v2 1/3] x86/fpu/xstate: Fix small issues before adding supervisor xstates Yu-cheng Yu
2019-12-20 20:19   ` Sebastian Andrzej Siewior
2020-01-06 18:15   ` [tip: x86/fpu] x86/fpu/xstate: Fix small issues tip-bot2 for Yu-cheng Yu
2019-12-12 21:08 ` [PATCH v2 2/3] x86/fpu/xstate: Make xfeature_is_supervisor()/xfeature_is_user() return bool Yu-cheng Yu
2019-12-20 20:19   ` Sebastian Andrzej Siewior
2019-12-20 20:33     ` Yu-cheng Yu
2020-01-06 18:15   ` [tip: x86/fpu] " tip-bot2 for Yu-cheng Yu
2019-12-12 21:08 ` [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails Yu-cheng Yu
2019-12-18 15:54   ` Sebastian Andrzej Siewior
2019-12-18 20:53     ` Yu-cheng Yu
2019-12-19 14:22       ` Sebastian Andrzej Siewior
2019-12-19 16:44         ` Yu-cheng Yu [this message]
2019-12-19 17:16           ` Sebastian Andrzej Siewior
2019-12-19 17:40             ` Yu-cheng Yu
2019-12-20 19:59               ` [PATCH] x86/fpu: Deacticate FPU state after failure during state load Sebastian Andrzej Siewior
2020-01-07 12:52                 ` [tip: x86/fpu] x86/fpu: Deactivate " tip-bot2 for Sebastian Andrzej Siewior
2020-01-07 20:41                   ` Andy Lutomirski
2020-01-07 20:38                     ` Yu-cheng Yu
2020-01-07 21:11                     ` Sebastian Andrzej Siewior
2020-01-08 11:45                       ` Borislav Petkov
2020-01-08 11:46                     ` Borislav Petkov
2019-12-20 20:16               ` [PATCH v2 3/3] x86/fpu/xstate: Invalidate fpregs when __fpu_restore_sig() fails Sebastian Andrzej Siewior
2019-12-20 20:32                 ` Yu-cheng Yu
  -- strict thread matches above, loose matches on Subject: below --
2019-12-05 18:26 [PATCH " Yu-cheng Yu
2019-12-07  4:38 ` [PATCH v2 " Yu-cheng Yu

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