Date: Tue, 20 Mar 2012 18:23:01 +0100 From: Ingo Molnar <mingo@...nel.org> To: Kees Cook <keescook@...omium.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Serge Hallyn <serge.hallyn@...onical.com>, linux-kernel@...r.kernel.org, Darren Hart <dvhart@...ux.intel.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Andrew Morton <akpm@...ux-foundation.org>, Jiri Kosina <jkosina@...e.cz>, "Eric W. Biederman" <ebiederm@...ssion.com>, David Howells <dhowells@...hat.com>, kernel-hardening@...ts.openwall.com, spender@...ecurity.net Subject: Re: [PATCH] futex: do not leak robust list to unprivileged process * Kees Cook <keescook@...omium.org> wrote: > On Tue, Mar 20, 2012 at 10:02 AM, Thomas Gleixner <tglx@...utronix.de> wrote: > > On Tue, 20 Mar 2012, Serge Hallyn wrote: > > > >> Quoting Kees Cook (keescook@...omium.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@...omium.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
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