LKML Archive on lore.kernel.org
 help / color / Atom feed
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: Wed, 18 Dec 2019 12:53:59 -0800
Message-ID: <587463c4e5fa82dff8748e5f753890ac390e993e.camel@intel.com> (raw)
In-Reply-To: <20191218155449.sk4gjabtynh67jqb@linutronix.de>

On Wed, 2019-12-18 at 16:54 +0100, Sebastian Andrzej Siewior wrote:
> On 2019-12-12 13:08:55 [-0800], Yu-cheng Yu wrote:
> > In __fpu_restore_sig(),'init_fpstate.xsave' and part of 'fpu->state.xsave'
> > are restored separately to xregs.  However, as stated in __cpu_invalidate_
> > fpregs_state(),
> > 
> >   Any code that clobbers the FPU registers or updates the in-memory
> >   FPU state for a task MUST let the rest of the kernel know that the
> >   FPU registers are no longer valid for this task.
> > 
> > and this code violates that rule.  Should the restoration fail, the other
> > task's context is corrupted.
> > 
> > This problem does not occur very often because copy_*_to_xregs() succeeds
> > most of the time.  
> 
> why "most of the time"? It should always succeed. We talk here about
> __fpu__restore_sig() correct? Using init_fpstate as part of restore
> process isn't the "default" case. If the restore _here_ fails then it
> fails.
> 
> >                    It occurs, for instance, in copy_user_to_fpregs_
> > zeroing() when the first half of the restoration succeeds and the other
> > half fails.  This can be triggered by running glibc tests, where a non-
> > present user stack page causes the XRSTOR to fail.
> 
> So if copy_user_to_fpregs_zeroing() fails then we go to the slowpath.
> Then we load the FPU register with copy_kernel_to_xregs_err().
> In the end they are either enabled (fpregs_mark_activate()) or cleared
> if it failed (fpu__clear()). Don't see here a problem.

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.
However, at this point, fpu_fpregs_owner_ctx has not been changed; it could
still be another task's FPU.  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.

> 
> Can you tell me which glibc test? I would like to reproduce this.
> 
> > The introduction of supervisor xstates and CET, while not contributing to
> > the problem, makes it more detectable.  After init_fpstate and the Shadow
> > Stack pointer have been restored to xregs, the XRSTOR from user stack
> > fails and fpu_fpregs_owner_ctx is not updated.  The task currently owning
> > fpregs then uses the corrupted Shadow Stack pointer and triggers a control-
> > protection fault.
> 
> So I don't need new HW with supervisor and CET? A plain KVM box with
> SSE2 and so should be enough?

What I do is, clone the whole glibc source, and run mutiple copies of
"make check".  In about 40 minutes or so, there are unexplained seg faults,
or a few control-protection faults (if you enable CET).  Please let me
know if more clarification is needed.

Thanks,
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 [this message]
2019-12-19 14:22       ` Sebastian Andrzej Siewior
2019-12-19 16:44         ` Yu-cheng Yu
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=587463c4e5fa82dff8748e5f753890ac390e993e.camel@intel.com \
    --to=yu-cheng.yu@intel.com \
    --cc=bigeasy@linutronix.de \
    --cc=bp@alien8.de \
    --cc=dave.hansen@linux.intel.com \
    --cc=fenghua.yu@intel.com \
    --cc=hpa@zytor.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=ravi.v.shankar@intel.com \
    --cc=riel@surriel.com \
    --cc=tglx@linutronix.de \
    --cc=tony.luck@intel.com \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git