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Date: Mon, 9 Mar 2015 17:24:04 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Borislav Petkov <bp@suse.de>
Cc: Dave Hansen <dave.hansen@intel.com>, Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pekka Riikonen <priikone@iki.fi>, Rik van Riel <riel@redhat.com>,
        Suresh Siddha <sbsiddha@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Quentin Casasnovas <quentin.casasnovas@oracle.com>
Subject: Re: [PATCH 1/1] x86/fpu: x86/fpu: avoid math_state_restore()
	without used_math() in __restore_xstate_sig()
Message-ID: <20150309162404.GA7833@redhat.com>
References: <54F74F59.5070107@intel.com> <20150305195127.GA12657@redhat.com> <20150307153824.GA25954@redhat.com> <20150307153844.GB25954@redhat.com> <20150309140712.GB12732@pd.tnic> <20150309143436.GA3526@redhat.com> <20150309151837.GC12732@pd.tnic>
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On 03/09, Borislav Petkov wrote:
>
> On Mon, Mar 09, 2015 at 03:34:36PM +0100, Oleg Nesterov wrote:
> > The patch only fixes the problem with irqs disabled, I tested this.
> >
> > The problem with fpu_init/XRSTORS is another thing...
>
> Yet another thing?! Oh boy.

Well, this is the same thinhg reported by Dave ;)

> So first Dave reported the #GP, which got fixed by Quentin's patch.

It is not fixed by Quentin's patch.

This patch "fixes" the problem in a sense that the kernel won't crash
after restore_fpu_checking() triggers #GP. Before this patch
do_general_protection()->fixup_exception() does not work in this case
and the kernel panics.

But restore_fpu_checking() should not trigger #GP (and fail).


And just in case... tip/x86/fpu still won't work even with the patch
from Quentin. Again, the kernel won't crash, but /sbin/init will be
killed by SIGSEGV I guess. Because restore_fpu_checking() will fail.

I'll change flush_thread() to rely on init_xstate_buf, I was going to
do this anyway. But this too doesn't fix the problem: fpu_finit() is
buggy on Dave's machine.

Oleg.