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") 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
--- kernel/cred.c | 9 +++++++++ kernel/ptrace.c | 10 ++++++++++ 2 files changed, 19 insertions(+)
--- a/kernel/cred.c +++ b/kernel/cred.c @@ -448,6 +448,15 @@ 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 @@ -323,6 +323,16 @@ 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) &&