linux-kernel - Re: + signal-always-clear-sa_restorer-on-execve.patch added to -mm tree

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Message-ID: <CAGXu5jLJwzxRtoU8A_gVbsPLdU6fyXZyBsr0EPxC9ngEhmDC0w@mail.gmail.com>
Date:	Mon, 11 Mar 2013 14:03:20 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	Ben Hutchings <ben@...adent.org.uk>,
	luis.henriques@...onical.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: + signal-always-clear-sa_restorer-on-execve.patch added to -mm tree

On Mon, Mar 11, 2013 at 2:01 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Mon, 11 Mar 2013 13:37:53 -0700 Kees Cook <keescook@...omium.org> wrote:
>
>> ...
>>
>
> (pop toasting undone)
>
>> > Subject: signal: always clear sa_restorer on execve
>> >
>> > When the new signal handlers are set up, 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().
>> >
>> > Based on what should be considered "secret" about addresses, it only
>> > matters across the exec not the fork (since the VMAs haven't changed until
>> > the exec).  But since exec sets SIG_DFL and keeps sa_restorer, this is
>> > where it should be fixed.
>>
>> A note for backporters: you'll likely want to change
>> __ARCH_HAS_SA_RESTORER to SA_RESTORER, since the former was recently
>> introduced. If not, this will apply but not actually do any good.
>
> I added this to the changelog, but I fear people won't read it!  Is
> there any clever way in which we can have one patch which will work OK
> in both old and new kernels?  I can't think of one...

Using just SA_RESTORER will work in both cases, but isn't "correct"
going forward. :(

-Kees

-- 
Kees Cook
Chrome OS Security
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