From: Kees Cook <keescook@chromium.org>
Date: Wed, 16 Jan 2019 18:31:09 +0000 (-0800)
Subject: Yama: Check for pid death before checking ancestry
X-Git-Tag: v5.0-rc3~36^2~1
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=9474f4e7cd71a633fa1ef93b7daefd44bbdfd482;p=thirdparty%2Flinux.git

Yama: Check for pid death before checking ancestry

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>
---

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index ffda91a4a1aaf..02514fe558b41 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -368,7 +368,9 @@ 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;