[PATCH 3.16 53/87] ptrace: restore smp_rmb() in __ptrace_may_access()

[Ben Hutchings]

3.16.75-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@xxxxxxxxxx>

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")
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/cred.c   |  9 +++++++++
 kernel/ptrace.c | 10 ++++++++++
 2 files changed, 19 insertions(+)

--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -439,6 +439,15 @@ int commit_creds(struct cred *new)
 		if (task->mm)
 			set_dumpable(task->mm, suid_dumpable);
 		task->pdeath_signal = 0;
+		/*
+		 * If a task drops privileges and becomes nondumpable,
+		 * the dumpability change must become visible before
+		 * the credential change; otherwise, a __ptrace_may_access()
+		 * racing with this change may be able to attach to a task it
+		 * shouldn't be able to attach to (as if the task had dropped
+		 * privileges without becoming nondumpable).
+		 * Pairs with a read barrier in __ptrace_may_access().
+		 */
 		smp_wmb();
 	}
 
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -324,6 +324,16 @@ static int __ptrace_may_access(struct ta
 	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) &&