get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove)

[Stefan Metzmacher]

Am 03.05.24 um 23:24 schrieb Linus Torvalds:
> On Fri, 3 May 2024 at 14:11, Al Viro <viro at zeniv.linux.org.uk> wrote:
>>
>> What we need is
>>          * promise that ep_item_poll() won't happen after eventpoll_release_file().
>> AFAICS, we do have that.
>>          * ->poll() not playing silly buggers.
> 
> No. That is not enough at all.
> 
> Because even with perfectly normal "->poll()", and even with the
> ep_item_poll() happening *before* eventpoll_release_file(), you have
> this trivial race:
> 
>    ep_item_poll()
>       ->poll()
> 
> and *between* those two operations, another CPU does "close()", and
> that causes eventpoll_release_file() to be called, and now f_count
> goes down to zero while ->poll() is running.
> 
> So you do need to increment the file count around the ->poll() call, I feel.
> 
> Or, alternatively, you'd need to serialize with
> eventpoll_release_file(), but that would need to be some sleeping lock
> held over the ->poll() call.
> 
>> As it is, dma_buf ->poll() is very suspicious regardless of that
>> mess - it can grab reference to file for unspecified interval.
> 
> I think that's actually much preferable to what epoll does, which is
> to keep using files without having reference counts to them (and then
> relying on magically not racing with eventpoll_release_file().

I think it's a very important detail that epoll does not take
real references. Otherwise an application level 'close()' on a socket
would not trigger a tcp disconnect, when an fd is still registered with
epoll.

I noticed that some parts of Samba currently rely on this when I tried
to convert tevent from epoll to IORING_OP_POLL_ADD (which takes a longer term reference)

And I guess there will be other applications also relying on the current epoll
behavior. That a closed fs automatically removes itself from epoll.

A short term reference just around ->poll() might be fine,
but please no reference via EPOLL_CTL_ADD.

Changing that can cause security problems in user space.

I haven't followed all details of this thread,
please ignore me if that's all clear already :-)

Thanks!
metze