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:	Wed, 18 Mar 2015 21:45:45 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Oleg Nesterov <oleg@...hat.com>, Andrey Wagin <avagin@...il.com>,
	Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Al Viro <viro@...iv.linux.org.uk>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
 delivered to 64-bit programs

On Wed, Mar 18, 2015 at 11:20:08AM -0700, Andy Lutomirski wrote:
> >
> > Basically the initial registers values are fetched with ptrace
> > when program is been dumped, then on restore we copy them back
> > into sigcontext. That said we don't mangle fs/gs anyhow simply
> > restore the values back from dump.
> 
> Wouldn't it be a little safer to have the kernel make you a sane
> sigcontext by raising a signal and catching it rather than writing a
> new one from scratch?  Grr, maybe the kernel should version its
> sigcontext structures.

I think it would, thanks for the point! I'll tune up our code.

> Getting fs from ptrace and restoring it to sigcontext is useless --
> ptrace handles fs correctly, but that sigcontext field is just
> padding.
> 
> Is CRIU at least reliably putting zero in the ss field?  If so, we
> could add a special case to translate zero to __USER_DS on restore.
> (And then I'll update my test case, and we'll have to document it,
> etc.)

Well, it should but need to check (we are tryin to not zeroify
things until really needed in a sake of speed).
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