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

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().

Link: https://lore.kernel.org/ahWxQ3JxdR5ff2qf@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Gregory Price <gourry@gourry.net>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Jann Horn <jannh@google.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/migrate.c b/mm/migrate.c
index d8090cdda4f987f7cb450b3eaadbf438437b111e..d9b23909d716c6b5e94e3266c39083f70a7e050f 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2555,24 +2555,29 @@ 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)