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 - 11:34:59 EST

On Thu, Apr 19, 2018 at 03:50:25PM +0300, Kirill Tkhai wrote:
> Hi, Al,
> commit 87b95ce0964c016ede92763be9c164e49f1019e9 is the first after which the below test crashes the kernel:
> Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Date: Sat Jan 10 19:01:08 2015 -0500
> switch the IO-triggering parts of umount to fs_pin
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> $modprobe dummy
> $while true
> do
> mkdir /var/run/netns
> touch /var/run/netns/init_net
> mount --bind /proc/1/ns/net /var/run/netns/init_net
> ip netns add foo
> ip netns exec foo ip link add dummy0 type dummy
> ip netns delete foo
> done

I can reproduce that, all right, and with a stack chain that
looks like this:
[77132.414912] pin_kill+0x81/0x150
[77132.424362] ? finish_wait+0x80/0x80
[77132.433917] mnt_pin_kill+0x1e/0x30
[77132.443829] cleanup_mnt+0x6b/0x70
[77132.453477] pin_kill+0x81/0x150
[77132.463064] ? finish_wait+0x80/0x80
[77132.472553] group_pin_kill+0x1a/0x30
[77132.481973] namespace_unlock+0x6f/0x80
[77132.491801] put_mnt_ns+0x1d/0x30
[77132.501258] free_nsproxy+0x17/0x90
[77132.510604] do_exit+0x2dc/0xb40
[77132.520146] ? handle_mm_fault+0xaa/0x1e0
[77132.529725] do_group_exit+0x3a/0xa0
[77132.539506] SyS_exit_group+0x10/0x10
with the top 4 entries repeated a lot. Those cleanup_mnt()
could be called from __cleanup_mnt(), delayed_mntput() or

__cleanup_mnt() is only fed to task_work_add(); no way in hell
would you get the call stack similar to that; it would be
called by task_work_run() from exit_task_work() from
do_exit(). Not in the evidence.

delayed_mntput() is only fed to schedule_delayed_work();
again, not a chance of having the call chain like that.

The one in mntput_no_expire() is a tail-call, with
mntput_no_expire() called from umount(2) and mntput()
(tail-calls both of them). The former is never called
from exit(2), so that call chain reads

pin_kill -> mntput or something tail-calling mntput -> mntput_no_expire ->
cleanup_mnt -> mnt_pin_kill -> pin_kill

Now, the thing called by pin_kill must be something passed to
init_fs_pin(), i.e. acct_pin_kill() or drop_mountpoint().
acct_pin_kill() ends with
static void acct_put(struct bsd_acct_struct *p)
if (atomic_long_dec_and_test(&p->count))
kfree_rcu(p, rcu);

IOW, no tail-call of mntput() in there. OTOH,
static void drop_mountpoint(struct fs_pin *p)
struct mount *m = container_of(p, struct mount, mnt_umount);
*does* have the tail-call, so this call chain must be
pin_kill -> drop_mountpoint -> mntput -> mntput_no_expire ->
cleanup_mnt -> mnt_pin_kill -> pin_kill

So far, so good, but if you look into mntput_no_expire() you see
if (likely(!(mnt->mnt.mnt_flags & MNT_INTERNAL))) {
struct task_struct *task = current;
if (likely(!(task->flags & PF_KTHREAD))) {
init_task_work(&mnt->mnt_rcu, __cleanup_mnt);
if (!task_work_add(task, &mnt->mnt_rcu, true))
if (llist_add(&mnt->mnt_llist, &delayed_mntput_list))
schedule_delayed_work(&delayed_mntput_work, 1);

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...