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Message-Id: <20240504092555.2071-1-hdanton@sina.com>
Date: Sat, 4 May 2024 17:25:55 +0800
From: Hillf Danton <hdanton@...a.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>,
Christian Konig <christian.koenig@....com>,
minhquangbui99@...il.com,
syzbot+045b454ab35fd82a35fb@...kaller.appspotmail.com,
syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes
On Fri, 3 May 2024 22:24:28 +0100 Al Viro wrote:
> On Fri, May 03, 2024 at 02:11:30PM -0700, Linus Torvalds wrote:
> > epoll is a mess, and does various invalid things in the name of
> > performance.
> >
> > Let's try to rein it in a bit. Something like this, perhaps?
>
> > +/*
> > + * The ffd.file pointer may be in the process of
> > + * being torn down due to being closed, but we
> > + * may not have finished eventpoll_release() yet.
> > + *
> > + * Technically, even with the atomic_long_inc_not_zero,
> > + * the file may have been free'd and then gotten
> > + * re-allocated to something else (since files are
> > + * not RCU-delayed, they are SLAB_TYPESAFE_BY_RCU).
>
> Can we get to ep_item_poll(epi, ...) after eventpoll_release_file()
> got past __ep_remove()? Because if we can, we have a worse problem -
Nope but mtx can help poll go before remove, see below.
> epi freed under us.
>
> If not, we couldn't possibly have reached ->release() yet, let
> alone freeing anything.
Actually poll can see a file with zero f_count, and LT's idea with
trival change survived the syzbot repro [1].
I think fput currently can race with epoll wrt f_count, and checking
it in dma-buf is necessary if his idea looks too aggressive.
wait_epoll() __fput()
do_epoll_wait() eventpoll_release_file()
ep_poll()
ep_send_events()
mutex_lock(&ep->mtx)
ep_item_poll()
vfs_poll()
mutex_unlock(&ep->mtx)
mutex_lock(&ep->mtx)
dispose = __ep_remove(ep, epi, true)
mutex_unlock(&ep->mtx)
[1] https://lore.kernel.org/lkml/000000000000f1c99d061798ac6d@google.com/
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