linux-kernel - Re: [PATCH] signal: always clear sa

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Message-ID: <87ip4xlo4y.fsf@xmission.com>
Date:	Mon, 11 Mar 2013 12:42:05 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Kees Cook <keescook@...omium.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Emese Revfy <re.emese@...il.com>,
	PaX Team <pageexec@...email.hu>
Subject: Re: [PATCH] signal: always clear sa_restorer on execve

Kees Cook <keescook@...omium.org> writes:

> On Mon, Mar 11, 2013 at 12:28 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> On 03/11, Kees Cook wrote:
>>>
>>> When the new signal handlers are set up for a fork, the location of
>>> sa_restorer is not cleared, leaking a parent process's address space
>>> location to children. This allows for a potential bypass of the parent's
>>> ASLR by examining the sa_restorer value returned when calling sigaction().
>>
>> I don't understand.
>>
>> fork() should not change restorer/etc, and the child has the same address
>> space anyway. There is no any leak and the patch can't make any difference
>> in this case because flush_signal_handlers() is not called by fork().
>
> I probably failed to explain this correctly. From the perspective of
> what should be considered "secret", it only matters across the exec,
> not the fork (since the VMAs haven't changed until the exec). But the
> info leak is easy to see, and this patch fixes it. As you say, since
> other things were reset, so should sa_restorer.

At the very least please correct the explanation in your patch
description.

Too often I have had seen a confused patch description, indicate
confusion elsewhere in the patch.  Let's make it easy for reviewers
and future bisectors to understand what is intended.

>>> @@ -485,6 +485,9 @@ flush_signal_handlers(struct task_struct *t, int force_default)
>>>               if (force_default || ka->sa.sa_handler != SIG_IGN)
>>>                       ka->sa.sa_handler = SIG_DFL;
>>>               ka->sa.sa_flags = 0;
>>> +#ifdef __ARCH_HAS_SA_RESTORER
>>> +             ka->sa.sa_restorer = NULL;
>>> +#endif

Also I am inclined to suggest this should be an inline function in a
header.
		clear_sa_restorer(ka);

Just so we don't litter the code with #ifdefs.

Eric
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