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

[Kirill Tkhai]

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: stable@xxxxxxxxxx
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

Flawless victory! Thanks.

Tested-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>

> ---
> 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;
>