ptrace: restore smp_rmb() in __ptrace_may_access()

Restore the read memory barrier in __ptrace_may_access() that was deleted
a couple years ago. Also add comments on this barrier and the one it pairs
with to explain why they're there (as far as I understand).

Fixes: bfedb589252c ("mm: Add a user_ns owner to mm_struct and fix ptrace permission checks")
Cc: stable@vger.kernel.org
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 02c6528ead5c..c9b4646ad375 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -323,6 +323,16 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
         return -EPERM;
 ok:
         rcu_read_unlock();
+        /*
+         * If a task drops privileges and becomes nondumpable (through a syscall
+         * like setresuid()) while we are trying to access it, we must ensure
+         * that the dumpability is read after the credentials; otherwise,
+         * we may be able to attach to a task that we shouldn't be able to
+         * attach to (as if the task had dropped privileges without becoming
+         * nondumpable).
+         * Pairs with a write barrier in commit_creds().
+         */
+        smp_rmb();
         mm = task->mm;
         if (mm &&
             ((get_dumpable(mm) != SUID_DUMP_USER) &&