linux-kernel - Re: [PATCH v3 2/7] x86/arch_prctl: Fix ARCH_GET_FS and ARCH_GET

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Date:	Fri, 8 Apr 2016 12:39:22 +0300
From:	Dmitry Safonov <0x7f454c46@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
	security@...nel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rudolf Marek <r.marek@...embler.cz>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, gorcunov@...nvz.org
Subject: Re: [PATCH v3 2/7] x86/arch_prctl: Fix ARCH_GET_FS and ARCH_GET_GS

2016-04-08 10:13 GMT+03:00 Ingo Molnar <mingo@...nel.org>:
>
> * Andy Lutomirski <luto@...nel.org> wrote:
>
>> ARCH_GET_FS and ARCH_GET_GS attempted to figure out the fsbase and
>> gsbase respectively from saved thread state.  This was wrong: fsbase
>> and gsbase live in registers while a thread is running, not in
>> memory.
>
> So I'm wondering, the current code looks totally broken,what user-space code can
> possibly use this? I checked glibc and Wine, and neither of them does. Wine uses
> ARCH_SET_GS and glibc uses ARCH_SET_FS, but that's all - neither actually tries to
> use the ARCH_GET_* reading APIs.

Just FYI, CRIU also does not use ARCH_GET_{FS,GS}, only ARCH_SET_{FS,GS}.
It gets them with the help of PTRACE_GETREGSET.

> So for backporting purposes I'd be much happier about simply returning -EINVAL or
> -ENOSYS, and we could re-introduce this code in v4.7.