[PATCH 4.16 068/279] proc: revalidate kernel thread inodes to root:root

[Greg Kroah-Hartman]

4.16-stable review patch.  If anyone has any objections, please let me know.

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

From: Alexey Dobriyan <adobri...@gmail.com>

[ Upstream commit 2e0ad552f5f8cd0fda02bc45fcd2b89821c62fd1 ]

task_dump_owner() has the following code:

        mm = task->mm;
        if (mm) {
                if (get_dumpable(mm) != SUID_DUMP_USER) {
                        uid = ...
                }
        }

Check for ->mm is buggy -- kernel thread might be borrowing mm
and inode will go to some random uid:gid pair.

Link: http://lkml.kernel.org/r/20180412220109.GA20978@avx2
Signed-off-by: Alexey Dobriyan <adobri...@gmail.com>
Cc: "Eric W. Biederman" <ebied...@xmission.com>
Signed-off-by: Andrew Morton <a...@linux-foundation.org>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.le...@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 fs/proc/base.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1693,6 +1693,12 @@ void task_dump_owner(struct task_struct
        kuid_t uid;
        kgid_t gid;
 
+       if (unlikely(task->flags & PF_KTHREAD)) {
+               *ruid = GLOBAL_ROOT_UID;
+               *rgid = GLOBAL_ROOT_GID;
+               return;
+       }
+
        /* Default to the tasks effective ownership */
        rcu_read_lock();
        cred = __task_cred(task);