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Date: Mon, 16 Mar 2015 09:11:53 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Al Viro <viro@zeniv.linux.org.uk>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
 delivered to 64-bit programs
Message-ID: <20150316081153.GA16393@gmail.com>
References: <cover.1426193719.git.luto@kernel.org>
 <405594361340a2ec32f8e2b115c142df0e180d8e.1426193719.git.luto@kernel.org>
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* Andy Lutomirski <luto@kernel.org> wrote:

> From: Andy Lutomirski <luto@amacapital.net>
> 
> The comment in the signal code says that apps can save/restore other 
> segments on their own.  It's true that apps can *save* SS on their 
> own, but there's no way for apps to restore it: SYSCALL effectively 
> resets SS to __USER_DS, so any value that user code tries to load 
> into SS gets lost on entry to sigreturn.
> 
> This recycles two padding bytes in the segment selector area for SS.
> 
> While we're at it, we need a second change to make this useful.  If 
> the signal we're delivering is caused by a bad SS value, saving that 
> value isn't enough.  We need to remove that bad value from the regs 
> before we try to deliver the signal.  Oddly, x32 already got this 
> right.

Note, it's the original i386 code that got this right, not the 
(relatively new) x32 code. I fixed this in the commit that I applied.

Thanks,

	Ingo