ptrace: restore smp_rmb() in __ptrace_may_access()
commit f6581f5b55141a95657ef5742cf6a6bfa20a109f upstream.
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index df06d2f..f447f1e3 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -322,6 +322,16 @@
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) &&