linux-kernel - Re: [PATCH] x86_64, entry: Fix out of bounds read on sysenter

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Date:	Sat, 01 Nov 2014 12:58:55 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Sebastian Lackner <sebastian@...-team.de>,
	Anish Bhatt <anish@...lsio.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chuck Ebbert <cebbert.lkml@...il.com>,
	Andy Lutomirski <luto@...capital.net>, stable@...r.kernel.org
Subject: Re: [PATCH] x86_64, entry: Fix out of bounds read on sysenter

Andy Lutomirski <luto@...capital.net> writes:
> Rusty noticed a Really Bad Bug (tm) in my NT fix.  The entry code
> reads out of bounds, causing the NT fix to be unreliable.  But, and
> this is much, much worse, if your stack is somehow just below the
> top of the direct map (or a hole), you read out of bounds and crash.
>
> Excerpt from the crash:
>
> [    1.129513] RSP: 0018:ffff88001da4bf88  EFLAGS: 00010296
>
>   2b:*    f7 84 24 90 00 00 00     testl  $0x4000,0x90(%rsp)
>
> That read is deterministically above the top of the stack.  I
> thought I even single-stepped through this code when I wrote it to
> check the offset, but I clearly screwed it up.
>
> Fixes 8c7aa698baca x86_64, entry: Filter RFLAGS.NT on entry from userspace
>
> Reported-by: Rusty Russell <rusty@...abs.org>
> Cc: stable@...r.kernel.org
> Signed-off-by: Andy Lutomirski <luto@...capital.net>

Tested-by: Rusty Russell <rusty@...tcorp.com.au>

Thanks for the fast response...
Rusty.
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