[PATCH 1/1] ptrace: Fix ->ptracer_cred handling for PTRACE_TRACEME

[Tyler Hicks]

From: Jann Horn <jannh at google.com>

Fix two issues:

When called for PTRACE_TRACEME, ptrace_link() would obtain an RCU
reference to the parent's objective credentials, then give that pointer
to get_cred().  However, the object lifetime rules for things like
struct cred do not permit unconditionally turning an RCU reference into
a stable reference.

PTRACE_TRACEME records the parent's credentials as if the parent was
acting as the subject, but that's not the case.  If a malicious
unprivileged child uses PTRACE_TRACEME and the parent is privileged, and
at a later point, the parent process becomes attacker-controlled
(because it drops privileges and calls execve()), the attacker ends up
with control over two processes with a privileged ptrace relationship,
which can be abused to ptrace a suid binary and obtain root privileges.

Fix both of these by always recording the credentials of the process
that is requesting the creation of the ptrace relationship:
current_cred() can't change under us, and current is the proper subject
for access control.

This change is theoretically userspace-visible, but I am not aware of
any code that it will actually break.

Fixes: 64b875f7ac8a ("ptrace: Capture the ptracer's creds not PT_PTRACE_CAP")
Signed-off-by: Jann Horn <jannh at google.com>
Acked-by: Oleg Nesterov <oleg at redhat.com>
Cc: stable at vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

CVE-2019-13272

(cherry picked from commit 6994eefb0053799d2e07cd140df6c2ea106c41ee)
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 kernel/ptrace.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 6f357f4fc859..eb012477910e 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -78,9 +78,7 @@ void __ptrace_link(struct task_struct *child, struct task_struct *new_parent,
  */
 static void ptrace_link(struct task_struct *child, struct task_struct *new_parent)
 {
-	rcu_read_lock();
-	__ptrace_link(child, new_parent, __task_cred(new_parent));
-	rcu_read_unlock();
+	__ptrace_link(child, new_parent, current_cred());
 }
 
 /**
-- 
2.7.4