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From: Thomas Gleixner <tglx@linutronix.de>
To: Dave Hansen <dave.hansen@intel.com>, Andrei Vagin <avagin@gmail.com>
Cc: Andrei Vagin <avagin@google.com>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 LKML <linux-kernel@vger.kernel.org>, x86@kernel.org, "H. Peter Anvin"
 <hpa@zytor.com>
Subject: Re: [PATCH] x86/fpu: verify xstate buffer size according with
 requested features
In-Reply-To: <5bddbb34-4081-494b-8c12-c2e70898a608@intel.com>
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 <b3e5456a-7113-4868-b8ce-020421e898ba@intel.com>
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 <54bcb902-0fab-4a53-8b8e-85b6e4484b03@intel.com> <87cytyfmd8.ffs@tglx>
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Date: Thu, 18 Jan 2024 23:11:40 +0100
Message-ID: <877ck6fg0z.ffs@tglx>
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On Thu, Jan 18 2024 at 14:02, Dave Hansen wrote:
> On 1/18/24 11:54, Thomas Gleixner wrote:
>> On Thu, Jan 18 2024 at 10:27, Dave Hansen wrote:
>>> If we have nice, reliable fault handling and then decide that we've got
>>> XRSTOR's running amok reading random memory all over the place that need
>>> a nicer error message, then we can add that code to predict the future.
>>> If our "predict the future" code goes wrong, then we lose an error
>>> message -- not a big deal.
>> After staring more at it, it's arguable to pass fpstate->user_size to
>> fault_in_readable() and ignore fx_sw->xstate_size completely.
>> 
>> That's a guaranteed to be reliable size which prevents endless loops
>> because arguably that's the maximum size which can be touched by XRSTOR,
>> no?
>
> I like it.  It takes fx_sw completely out of the picture, which was the
> root of the problem in the first place.

Correct.

I really don't care about the esoteric case where this might
theoretically result in a unjustified application abort.

You really need to twist your brain around 6 corners and then squint
twice to construct that case. Of course syzcaller might trigger it, but
fuzzing the sigreturn frame is a #GP, #PF and whatever lottery anyway.