Messages in this thread | | | Date | Mon, 6 May 2024 19:46:08 +0200 | Subject | Re: get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) | From | Stefan Metzmacher <> |
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Am 03.05.24 um 23:24 schrieb Linus Torvalds: > On Fri, 3 May 2024 at 14:11, Al Viro <viro@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
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