Re: [PATCH] dmabuf: fix dmabuf file poll uaf issue

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Am 12.04.24 um 08:19 schrieb zhiguojiang:
[SNIP]
-> Here task 2220 do epoll again where internally it will get/put then start to free twice and lead to final crash.

Here is the basic flow:

1. Thread A install the dma_buf_fd via dma_buf_export, the fd refcount is 1

2. Thread A add the fd to epoll list via epoll_ctl(EPOLL_CTL_ADD)

3. After use the dma buf, Thread A close the fd, then here fd refcount is 0,
  and it will run __fput finally to release the file

Stop, that isn't correct.

The fs layer which calls dma_buf_poll() should make sure that the file can't go away until the function returns.

Then inside dma_buf_poll() we add another reference to the file while installing the callback:

                        /* Paired with fput in dma_buf_poll_cb */
                        get_file(dmabuf->file);

This reference is only dropped after the callback is completed in dma_buf_poll_cb():

        /* Paired with get_file in dma_buf_poll */
        fput(dmabuf->file);

So your explanation for the issue just seems to be incorrect.


4. Here Thread A not do epoll_ctl(EPOLL_CTL_DEL) manunally, so it still resides in one epoll_list.   Although __fput will call eventpoll_release to remove the file from binded epoll list,
  but it has small time window where Thread B jumps in.

Well if that is really the case then that would then be a bug in epoll_ctl().


5. During the small window, Thread B do the poll action for dma_buf_fd, where it will fget/fput for the file,   this means the fd refcount will be 0 -> 1 -> 0, and it will call __fput again.
  This will lead to __fput twice for the same file.

6. So the potenial fix is use get_file_rcu which to check if file refcount already zero which means under free.
  If so, we just return and no need to do the dma_buf_poll.

Well to say it bluntly as far as I can see this suggestion is completely nonsense.

When the reference to the file goes away while dma_buf_poll() is executed then that's a massive bug in the caller of that function.

Regards,
Christian.


Here is the race condition:

Thread A Thread B
dma_buf_export
fd_refcount is 1
epoll_ctl(EPOLL_ADD)
add dma_buf_fd to epoll list
close(dma_buf_fd)
fd_refcount is 0
__fput
dma_buf_poll
fget
fput
fd_refcount is zero again

Thanks





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