linux-kernel - Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs

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Date:	Mon, 16 Mar 2015 09:11:53 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	Andi Kleen <andi@...stfloor.org>, "H. Peter Anvin" <hpa@...or.com>,
	Al Viro <viro@...iv.linux.org.uk>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
 delivered to 64-bit programs


* Andy Lutomirski <luto@...nel.org> wrote:

> From: Andy Lutomirski <luto@...capital.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
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