[PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()

[Jann Horn]

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: sta...@vger.kernel.org
Signed-off-by: Jann Horn <ja...@google.com>
---
(I have no clue whatsoever what the relevant tree for this is, but I
guess Oleg is the relevant maintainer?)

 kernel/cred.c   |  9 +++++++++
 kernel/ptrace.c | 10 ++++++++++
 2 files changed, 19 insertions(+)

diff --git a/kernel/cred.c b/kernel/cred.c
index 45d77284aed0..07e069d00696 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -450,6 +450,15 @@ 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();
        }
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 5710d07e67cf..e54452c2954b 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -324,6 +324,16 @@ 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) &&
-- 
2.22.0.rc1.257.g3120a18244-goog