linux-kernel - [PATCH 3.16 159/305] mount: Retest MNT_LOCKED in do

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Message-ID: <lsq.1549201508.331999817@decadent.org.uk>
Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Al Viro" <viro@...IV.linux.org.uk>
Subject: [PATCH 3.16 159/305] mount: Retest MNT_LOCKED in do_umount

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

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

From: "Eric W. Biederman" <ebiederm@...ssion.com>

commit 25d202ed820ee347edec0bf3bf553544556bf64b upstream.

It was recently pointed out that the one instance of testing MNT_LOCKED
outside of the namespace_sem is in ksys_umount.

Fix that by adding a test inside of do_umount with namespace_sem and
the mount_lock held.  As it helps to fail fails the existing test is
maintained with an additional comment pointing out that it may be racy
because the locks are not held.

Reported-by: Al Viro <viro@...IV.linux.org.uk>
Fixes: 5ff9d8a65ce8 ("vfs: Lock in place mounts from more privileged users")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/namespace.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1423,8 +1423,13 @@ static int do_umount(struct mount *mnt,
 
 	namespace_lock();
 	lock_mount_hash();
-	event++;
 
+	/* Recheck MNT_LOCKED with the locks held */
+	retval = -EINVAL;
+	if (mnt->mnt.mnt_flags & MNT_LOCKED)
+		goto out;
+
+	event++;
 	if (flags & MNT_DETACH) {
 		if (!list_empty(&mnt->mnt_list))
 			umount_tree(mnt, UMOUNT_PROPAGATE);
@@ -1438,6 +1443,7 @@ static int do_umount(struct mount *mnt,
 			retval = 0;
 		}
 	}
+out:
 	unlock_mount_hash();
 	namespace_unlock();
 	return retval;
@@ -1484,7 +1490,7 @@ SYSCALL_DEFINE2(umount, char __user *, n
 		goto dput_and_out;
 	if (!check_mnt(mnt))
 		goto dput_and_out;
-	if (mnt->mnt.mnt_flags & MNT_LOCKED)
+	if (mnt->mnt.mnt_flags & MNT_LOCKED) /* Check optimistically */
 		goto dput_and_out;
 	retval = -EPERM;
 	if (flags & MNT_FORCE && !capable(CAP_SYS_ADMIN))