Re: [BUG] fs/super: a possible sleep-in-atomic bug in put_super

From: Al Viro
Date: Fri Oct 06 2017 - 08:19:49 EST

On Fri, Oct 06, 2017 at 04:59:18PM +0800, Jia-Ju Bai wrote:
> According to fs/super.c, the kernel may sleep under a spinlock.
> The function call path is:
> put_super (acquire the spinlock)
> __put_super
> destroy_super
> list_lru_destroy
> list_lru_unregister
> mutex_lock --> may sleep
> memcg_get_cache_ids
> down_read --> may sleep
> This bug is found by my static analysis tool and my code review.

Invariant to watch is this: s->s_active > 0 => s->s_count > 0. In other
words, anything that passes from __put_super() to destroy_super() has
already been through deactivate_locked_super() to list_lru_destroy().

And list_lru_destroy() called twice without list_lru_init() between
those will quietly do nothing on the second call.

All other callers of destroy_super() are from alloc_super()/sget_userns().
The former is the only place where instances are created, the latter is
the only caller of the former. Direct calls of destroy_super() in there
a) happen only to instances that had not been visible in any
shared data structures yet and
b) are done with no spinlocks held.

struct super_block has probably the most complex lifecycle in the entire VFS.
These days the nastiness is fortunately limited to fs/super.c guts, but
it's very definitely still there. I've posted sketches of description on
fsdevel several times, but never managed to turn that into coherent text ;-/