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

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Message-ID: <87cytyfmd8.ffs@tglx>
Date: Thu, 18 Jan 2024 20:54:43 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Dave Hansen <dave.hansen@...el.com>, 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 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?

Thanks,

        tglx