Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Tue, 20 Mar 2012 10:11:02 -0700
From: Kees Cook <>
To: Thomas Gleixner <>
Cc: Serge Hallyn <>,, 
	Darren Hart <>, Peter Zijlstra <>, 
	Andrew Morton <>, Jiri Kosina <>, 
	"Eric W. Biederman" <>, David Howells <>,,,
Subject: Re: [PATCH] futex: do not leak robust list to unprivileged process

On Tue, Mar 20, 2012 at 10:02 AM, Thomas Gleixner <> wrote:
> On Tue, 20 Mar 2012, Serge Hallyn wrote:
>> Quoting Kees Cook (
>> > It was possible to extract the robust list head address from a setuid
>> > process if it had used set_robust_list(), allowing an ASLR info leak. This
>> > changes the permission checks to be the same as those used for similar
>> > info that comes out of /proc.
>> >
>> > Running a setuid program that uses robust futexes would have had:
>> >   cred->euid != pcred->euid
>> >   cred->euid == pcred->uid
>> > so the old permissions check would allow it. I'm not aware of any setuid
>> > programs that use robust futexes, so this is just a preventative measure.
>> >
>> > (This patch is based on changes from grsecurity.)
>> >
>> > Signed-off-by: Kees Cook <>
>> I like the change.  Much cleaner.  I'm not 100% sure though that
>> there are no legitimate cases of robust futexes use which would now
>> be forbidden.  (Explicitly cc:ing Ingo)
> get_robust_list is not necessary for robust futexes. There is no
> reference to get_robust_list in glibc.
> I really wonder why we have this syscall at all.

The documentation I found yesterday while looking at this was:

Which says "The system call is only available for debugging purposes
and is not needed for normal operations. Both system calls are not
available to application programs as functions; they can be called
using the syscall(3) function."

Dropping the syscall entirely would certainly make it secure. ;)


Kees Cook
ChromeOS Security

Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.