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Subject[PATCH 3.16 53/87] ptrace: restore smp_rmb() in __ptrace_may_access()
3.16.75-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Jann Horn <jannh@google.com>

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@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: Ben Hutchings <ben@decadent.org.uk>
---
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) &&
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