[kernel-hardening] Re: [PATCH] futex: do not leak robust list to unprivileged process

[Ingo Molnar <mingo@kernel.org>]

* Kees Cook <keescook@chromium.org> wrote:

> On Tue, Mar 20, 2012 at 10:02 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Tue, 20 Mar 2012, Serge Hallyn wrote:
> >
> >> Quoting Kees Cook (keescook@chromium.org):
> >> > 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 <keescook@chromium.org>
> >>
> >> 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: 
> http://linux.die.net/man/2/get_robust_list
> 
> 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. 
> ;)

The thinking was API completeness. In general it's possible for 
a sufficiently privileged task to figure out all the state of a 
task. We can query timers, fds - the robust list is such a 
resource as well. The information leakage was obviously not 
intended.

Thanks,

	Ingo