[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@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 2d514487faf1 ("security: Yama LSM")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
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