linux-kernel - Re: [BUG] fs/super: a possible sleep-in-atomic bug in put

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Date:   Fri, 6 Oct 2017 13:19:30 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Jia-Ju Bai <baijiaju1990@....com>
Cc:     torbjorn.lindh@...ta.se, rgooch@...f.csiro.au,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG] fs/super: a possible sleep-in-atomic bug in put_super

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 ;-/