linux-kernel - Re: [tip:x86/urgent] x86/tls: Don't validate lm in set_thread

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Date:	Thu, 18 Dec 2014 08:59:28 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	linux-kernel@...r.kernel.org, luto@...capital.net,
	torvalds@...ux-foundation.org, mingo@...nel.org,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/tls: Don't validate lm in set_thread_area()
 after all

On 12/18/2014 03:16 AM, tip-bot for Andy Lutomirski wrote:
> Commit-ID:  3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0
> Gitweb:     http://git.kernel.org/tip/3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0
> Author:     Andy Lutomirski <luto@...capital.net>
> AuthorDate: Wed, 17 Dec 2014 14:48:30 -0800
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Thu, 18 Dec 2014 12:12:26 +0100
> 
> x86/tls: Don't validate lm in set_thread_area() after all
> 
> It turns out that there's a lurking ABI issue.  GCC, when
> compiling this in a 32-bit program:
> 
> struct user_desc desc = {
> 	.entry_number    = idx,
> 	.base_addr       = base,
> 	.limit           = 0xfffff,
> 	.seg_32bit       = 1,
> 	.contents        = 0, /* Data, grow-up */
> 	.read_exec_only  = 0,
> 	.limit_in_pages  = 1,
> 	.seg_not_present = 0,
> 	.useable         = 0,
> };
> 
> will leave .lm uninitialized.  This means that anything in the
> kernel that reads user_desc.lm for 32-bit tasks is unreliable.
> 

No, it won't.  However, if you initialize this dynamically field by
field rather than as an initializer, then you are correct.

	-hpa

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