linux-kernel - [PATCH 3.16 047/202] Yama: Check for pid death before checking ancestry

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Message-ID: <lsq.1556377989.886211100@decadent.org.uk>
Date:   Sat, 27 Apr 2019 16:13:09 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Oleg Nesterov" <oleg@...hat.com>,
        "James Morris" <james.morris@...rosoft.com>,
        syzbot+a9ac39bf55329e206219@...kaller.appspotmail.com,
        "Kees Cook" <keescook@...omium.org>
Subject: [PATCH 3.16 047/202] Yama: Check for pid death before checking
 ancestry

3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Kees Cook <keescook@...omium.org>

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@...kaller.appspotmail.com
Fixes: 2d514487faf1 ("security: Yama LSM")
Suggested-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
Signed-off-by: James Morris <james.morris@...rosoft.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 security/yama/yama_lsm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -299,7 +299,9 @@ int yama_ptrace_access_check(struct task
 			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;