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:	Fri, 13 Mar 2015 17:13:38 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	Ingo Molnar <mingo@...nel.org>, 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>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
 delivered to 64-bit programs

On Thu, Mar 12, 2015 at 01:57:51PM -0700, Andy Lutomirski 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.
> 
> I suspect that 64-bit programs that try to run 16-bit code and use
> signals will have a lot of trouble without this.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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