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Date: Wed, 17 Jan 2024 11:34:41 -0800
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Subject: Re: [PATCH] x86/fpu: verify xstate buffer size according with
 requested features
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To: Andrei Vagin <avagin@google.com>, Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
 Dave Hansen <dave.hansen@linux.intel.com>,
 LKML <linux-kernel@vger.kernel.org>
Cc: x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
References: <20240116234901.3238852-1-avagin@google.com>
From: Dave Hansen <dave.hansen@intel.com>
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In-Reply-To: <20240116234901.3238852-1-avagin@google.com>
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.. adding LKML.  More context here:

https://lore.kernel.org/all/20240116234901.3238852-1-avagin@google.com/

On 1/16/24 15:49, Andrei Vagin wrote:
> +	/* xstate_size has to fit all requested components. */
> +	if (fx_sw->xstate_size != fpstate->user_size) {
> +		int min_xstate_size =
> +			xstate_calculate_size(fx_sw->xfeatures, false);
> +
> +		if (min_xstate_size < 0 ||
> +		    fx_sw->xstate_size < min_xstate_size ||
> +		    fx_sw->xstate_size > fpstate->user_size)
> +			goto setfx;
> +	}

The bug here is that the buffer from userspace is garbage and the (XSAVE
XSTATE_BV) metadata doesn't match the size of the buffer.  Right?

This proposed fix just checks another piece of user-supplied metadata
instead: fx_sw->xstate_size.

Can't userspace just provide more bad data there and end up with the
same problem?

Seems like the real problem here is that the fault_in_readable() doesn't
match the XRSTOR.  It's going to continue to be a problem as long as we
don't know what memory XRSTOR tried to access.  We can try all day long
to precalculate what XRSTOR _will_ do, but that seems a bit silly
because the CPU knows where the fault happened.  It told us in CR2 and
all we have to do is plumb that back to fault_in_readable().

It would take a little XSTATE_OP() munging to pass something back other
than 'err', but that doesn't seem insurmountable.

Anybody have better ideas?