linux-kernel - Re: [PATCH] x86/fpu: verify xstate buffer size according with requested features

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Date: Thu, 18 Jan 2024 14:02:08 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Thomas Gleixner <tglx@...utronix.de>, Andrei Vagin <avagin@...il.com>
Cc: Andrei Vagin <avagin@...gle.com>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/fpu: verify xstate buffer size according with
 requested features

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.