Yama: Check for pid death before checking ancestry

commit 9474f4e7cd71a633fa1ef93b7daefd44bbdfd482 upstream.

It's possible that a pid has died before we take the rcu lock, in which
case we can't walk the ancestry list as it may be detached. Instead, check
for death first before doing the walk.

Reported-by: syzbot+a9ac39bf55329e206219@syzkaller.appspotmail.com
Fixes: 2d514487faf1 ("security: Yama LSM")
Cc: stable@vger.kernel.org
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index 0038834b558eb6a86e6d5330ea6011a2bafa4bf9..6da5316036143b19d61ff00b50753cc57ee3fb74 100644 (file)
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -299,7 +299,9 @@ int yama_ptrace_access_check(struct task_struct *child,
                        break;
                case YAMA_SCOPE_RELATIONAL:
                        rcu_read_lock();
-                       if (!task_is_descendant(current, child) &&
+                       if (!pid_alive(child))
+                               rc = -EPERM;
+                       if (!rc && !task_is_descendant(current, child) &&
                            !ptracer_exception_found(current, child) &&
                            !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
                                rc = -EPERM;