Re: [bisected] Stack overflow after fs: "switch the IO-triggering parts of umount to fs_pin" (was net namespaces kernel stack overflow)

From: Al Viro
Date: Thu Apr 19 2018 - 12:44:35 EST

On Thu, Apr 19, 2018 at 04:34:48PM +0100, Al Viro wrote:

> IOW, we only get there if our vfsmount was an MNT_INTERNAL one.
> So we have mnt->mnt_umount of some MNT_INTERNAL mount found in
> ->mnt_pins of some other mount. Which, AFAICS, means that
> it used to be mounted on that other mount. How the hell can
> that happen?
> It looks like you somehow get a long chain of MNT_INTERNAL mounts
> stacked on top of each other, which ought to be prevented by
> mnt_flags &= ~MNT_INTERNAL_FLAGS;
> in do_add_mount(). Nuts...

Arrrrrgh... Nuts is right - clone_mnt() preserves the sodding
MNT_INTERNAL, with obvious results.

netns is related to the problem, by exposing MNT_INTERNAL mounts
(in /proc/*/ns/*) for mount --bind to copy and attach to the
tree. AFAICS, the minimal reproducer is

touch /tmp/a
unshare -m sh -c 'for i in `seq 10000`; do mount --bind /proc/1/ns/net /tmp/a; done'

(and it can be anything in /proc/*/ns/*, really)

I think the fix should be along the lines of the following:

Don't leak MNT_INTERNAL away from internal mounts

We want it only for the stuff created by SB_KERNMOUNT mounts, *not* for
their copies.

Cc: stable@xxxxxxxxxx
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
diff --git a/fs/namespace.c b/fs/namespace.c
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1089,7 +1089,8 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
goto out_free;

- mnt->mnt.mnt_flags = old->mnt.mnt_flags & ~(MNT_WRITE_HOLD|MNT_MARKED);
+ mnt->mnt.mnt_flags = old->mnt.mnt_flags;
+ mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL);
/* Don't allow unprivileged users to change mount flags */
if (flag & CL_UNPRIVILEGED) {
mnt->mnt.mnt_flags |= MNT_LOCK_ATIME;