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Message-ID: <20191219142217.axgpqlb7zzluoxnf@linutronix.de>
Date: Thu, 19 Dec 2019 15:22:17 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Yu-cheng Yu <yu-cheng.yu@...el.com>
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 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.
> > 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.
Okay. Can you please try the above and if not, I try that glibc thing myself.
> Thanks,
> Yu-cheng
Sebastian
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