Re: [PATCH bpf] xsk: fix crash on double free in buffer pool

["Björn Töpel" <bjorn.topel@gmail.com>]

On Thu, 11 Nov 2021 at 08:57, Magnus Karlsson <magnus.karlsson@gmail.com> wrote:
>
> From: Magnus Karlsson <magnus.karlsson@intel.com>
>
> Fix a crash in the buffer pool allocator when a buffer is double
> freed. It is possible to trigger this behavior not only from a faulty
> driver, but also from user space like this: Create a zero-copy AF_XDP
> socket. Load an XDP program that will issue XDP_DROP for all
> packets. Put the same umem buffer into the fill ring multiple times,
> then bind the socket and send some traffic. This will crash the kernel
> as the XDP_DROP action triggers one call to xsk_buff_free()/xp_free()
> for every packet dropped. Each call will add the corresponding buffer
> entry to the free_list and increase the free_list_cnt. Some entries
> will have been added multiple times due to the same buffer being
> freed. The buffer allocation code will then traverse this broken list
> and since the same buffer is in the list multiple times, it will try
> to delete the same buffer twice from the list leading to a crash.
>
> The fix for this is just to test that the buffer has not been added
> before in xp_free(). If it has been, just return from the function and
> do not put it in the free_list a second time.
>
> Note that this bug was not present in the code before the commit
> referenced in the Fixes tag. That code used one list entry per
> allocated buffer, so multiple frees did not have any side effects. But
> the commit below optimized the usage of the pool and only uses a
> single entry per buffer in the umem, meaning that multiple
> allocations/frees of the same buffer will also only use one entry,
> thus leading to the problem.
>
> Fixes: 47e4075df300 ("xsk: Batched buffer allocation for the pool")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>

Acked-by: Björn Töpel <bjorn@kernel.org>