[PATCH 3.8 09/13] mnt: Move the test for MNT_LOCK_READONLY from change_mount_flags into do_remount

From: Kamal Mostafa
Date: Mon Aug 25 2014 - 12:58:04 EST -stable review patch. If anyone has any objections, please let me know.


From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

commit 07b645589dcda8b7a5249e096fece2a67556f0f4 upstream.

There are no races as locked mount flags are guaranteed to never change.

Moving the test into do_remount makes it more visible, and ensures all
filesystem remounts pass the MNT_LOCK_READONLY permission check. This
second case is not an issue today as filesystem remounts are guarded
by capable(CAP_DAC_ADMIN) and thus will always fail in less privileged
mount namespaces, but it could become an issue in the future.

Acked-by: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
fs/namespace.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index ddbd5bc..9171ac3 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1740,9 +1740,6 @@ static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
if (readonly_request == __mnt_is_readonly(mnt))
return 0;

- if (mnt->mnt_flags & MNT_LOCK_READONLY)
- return -EPERM;
if (readonly_request)
error = mnt_make_readonly(real_mount(mnt));
@@ -1771,6 +1768,16 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
if (path->dentry != path->mnt->mnt_root)
return -EINVAL;

+ /* Don't allow changing of locked mnt flags.
+ *
+ * No locks need to be held here while testing the various
+ * MNT_LOCK flags because those flags can never be cleared
+ * once they are set.
+ */
+ if ((mnt->mnt.mnt_flags & MNT_LOCK_READONLY) &&
+ !(mnt_flags & MNT_READONLY)) {
+ return -EPERM;
+ }
err = security_sb_remount(sb, data);
if (err)
return err;

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