futex: Do not leak robust list to unprivileged process

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>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Serge E. Hallyn <serge.hallyn@canonical.com>
Cc: kernel-hardening@lists.openwall.com
Cc: spender@grsecurity.net
Link: http://lkml.kernel.org/r/20120319231253.GA20893@www.outflux.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 13 insertions, 23 deletions

diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index 5f9e689dc8f0..a9642d528630 100644
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -10,6 +10,7 @@
 #include <linux/compat.h>
 #include <linux/nsproxy.h>
 #include <linux/futex.h>
+#include <linux/ptrace.h>
 
 #include <asm/uaccess.h>
 
@@ -136,40 +137,29 @@ compat_sys_get_robust_list(int pid, compat_uptr_t __user *head_ptr,
 {
         struct compat_robust_list_head __user *head;
         unsigned long ret;
-        const struct cred *cred = current_cred(), *pcred;
+        struct task_struct *p;
 
         if (!futex_cmpxchg_enabled)
                 return -ENOSYS;
 
+        rcu_read_lock();
+
+        ret = -ESRCH;
         if (!pid)
-                head = current->compat_robust_list;
+                p = current;
         else {
-                struct task_struct *p;
-
-                ret = -ESRCH;
-                rcu_read_lock();
                 p = find_task_by_vpid(pid);
                 if (!p)
                         goto err_unlock;
-                ret = -EPERM;
-                pcred = __task_cred(p);
-                /* If victim is in different user_ns, then uids are not
-                   comparable, so we must have CAP_SYS_PTRACE */
-                if (cred->user->user_ns != pcred->user->user_ns) {
-                        if (!ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
-                                goto err_unlock;
-                        goto ok;
-                }
-                /* If victim is in same user_ns, then uids are comparable */
-                if (cred->euid != pcred->euid &&
-                    cred->euid != pcred->uid &&
-                    !ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
-                        goto err_unlock;
-ok:
-                head = p->compat_robust_list;
-                rcu_read_unlock();
         }
 
+        ret = -EPERM;
+        if (!ptrace_may_access(p, PTRACE_MODE_READ))
+                goto err_unlock;
+
+        head = p->compat_robust_list;
+        rcu_read_unlock();
+
         if (put_user(sizeof(*head), len_ptr))
                 return -EFAULT;
         return put_user(ptr_to_compat(head), head_ptr);