[PATCH] mm/migrate: find_mm_struct: fix race between security checks and suid exec

[Oleg Nesterov <oleg@redhat.com>]

The target task can execute a setuid binary between ptrace_may_access()
and get_task_mm(). Protect this critical section with exec_update_lock.

I don't think cpuset_mems_allowed(task) should be called under
exec_update_lock, but this patch just tries to add the minimal fix.

Perhaps we can later add a common helper which can be used by
find_mm_struct() and kernel_migrate_pages().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 mm/migrate.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 8a64291ab5b4..e4cf664fb161 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2557,24 +2557,29 @@ static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
 	}
 
 	task = find_get_task_by_vpid(pid);
-	if (!task) {
+	if (!task)
 		return ERR_PTR(-ESRCH);
-	}
 
+	if (down_read_killable(&task->signal->exec_update_lock)) {
+		mm = ERR_PTR(-EINTR);
+		goto out;
+	}
 	/*
 	 * Check if this process has the right to modify the specified
 	 * process. Use the regular "ptrace_may_access()" checks.
 	 */
 	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
 		mm = ERR_PTR(-EPERM);
-		goto out;
+		goto unlock;
 	}
 
 	mm = ERR_PTR(security_task_movememory(task));
 	if (IS_ERR(mm))
-		goto out;
+		goto unlock;
 	*mem_nodes = cpuset_mems_allowed(task);
 	mm = get_task_mm(task);
+unlock:
+	up_read(&task->signal->exec_update_lock);
 out:
 	put_task_struct(task);
 	if (!mm)
-- 
2.52.0