[PATCH] Yama: Check for pid death before checking ancestry

[Kees Cook]

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 at syzkaller.appspotmail.com
Fixes: 2d514487faf1 ("security: Yama LSM")
Cc: stable at vger.kernel.org
Suggested-by: Oleg Nesterov <oleg at redhat.com>
Signed-off-by: Kees Cook <keescook at chromium.org>
---
James, can you please send this to Linus in your -fixes tree?
---
 security/yama/yama_lsm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index ffda91a4a1aa..02514fe558b4 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;
-- 
2.17.1


-- 
Kees Cook