proc: revalidate kernel thread inodes to root:root

BugLink: http://bugs.launchpad.net/bugs/1794889
[ 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 <adobriyan@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 948602fb2bc97741b75feb7efc4231ec88ed72fc..bd602299385c185022cbbcba1a36fc0134beaea0 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1704,6 +1704,12 @@ void task_dump_owner(struct task_struct *task, umode_t mode,
        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);