linux-kernel - Re: RELEASE BLOCKER: Linux doesn't follow x86/x86-64 ABI wrt direction flag

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Date:	Thu, 06 Mar 2008 19:13:20 +0100
From:	Paolo Bonzini <bonzini@....org>
To:	Olivier Galibert <galibert@...ox.com>,
	Joe Buck <Joe.Buck@...opsys.COM>,
	Paolo Bonzini <bonzini@....org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Chris Lattner <clattner@...le.com>,
	Michael Matz <matz@...e.de>,
	Richard Guenther <richard.guenther@...il.com>,
	Jan Hubicka <hubicka@....cz>,
	Aurelien Jarno <aurelien@...el32.net>,
	linux-kernel@...r.kernel.org, gcc@....gnu.org
Subject: Re: RELEASE BLOCKER: Linux doesn't follow x86/x86-64 ABI wrt direction
   flag

Olivier Galibert wrote:
> On Thu, Mar 06, 2008 at 09:58:41AM -0800, Joe Buck wrote:
>> If the kernel allows state to leak from one process to another,
>> for example from a process running as root to a process running as an
>> ordinary user, it's a bug, with possible security implications.
> 
> I don't think that it is relevant in your case.  If you have the
> signal handler in something that does not share the VM with the
> interrupted thread, you will have a context switch which is supposed
> to store the direction flag and restore the one from the handling
> thread.  If you share the VM there is no context switch but you have
> access to the exact same memory with the exact same rights, making the
> leak irrelevant.

A process can send a signal via kill.  IOW, a malicious process can 
*control when the process would be interrupted* in order to get it into 
the signal handler with DF=1.

Paolo
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