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 0309f2111c703b499134d3217d0a3735cdb5f2a0..5367f854fadc143b30d7c6924b43308b058385a4 100644 (file)
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -359,7 +359,9 @@ static 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;