On 19.04.2018 19:44, Al Viro wrote:
> 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: sta...@kernel.org
> Signed-off-by: Al Viro <v...@zeniv.linux.org.uk>
Flawless victory! Thanks.
Tested-by: Kirill Tkhai <ktk...@virtuozzo.com>
> ---
> 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;
>
