netdev - Re: [PATCH bpf] xsk: fix missing validation for skb and unaligned mode

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Date:   Fri, 18 Jun 2021 15:43:49 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf] xsk: fix missing validation for skb and unaligned mode

On Thu, 17 Jun 2021 at 11:23, Magnus Karlsson <magnus.karlsson@...il.com> wrote:
>
> From: Magnus Karlsson <magnus.karlsson@...el.com>
>
> Fix a missing validation of a Tx descriptor when executing in skb mode
> and the umem is in unaligned mode. A descriptor could point to a
> buffer straddling the end of the umem, thus effectively tricking the
> kernel to read outside the allowed umem region. This could lead to a
> kernel crash if that part of memory is not mapped.
>
> In zero-copy mode, the descriptor validation code rejects such
> descriptors by checking a bit in the DMA address that tells us if the
> next page is physically contiguous or not. For the last page in the
> umem, this bit is not set, therefore any descriptor pointing to a
> packet straddling this last page boundary will be rejected. However,
> the skb path does not use this bit since it copies out data and can do
> so to two different pages. (It also does not have the array of DMA
> address, so it cannot even store this bit.) The code just returned
> that the packet is always physically contiguous. But this is
> unfortunately also returned for the last page in the umem, which means
> that packets that cross the end of the umem are being allowed, which
> they should not be.
>
> Fix this by introducing a check for this in the SKB path only, not
> penalizing the zero-copy path.
>
> Fixes: 2b43470add8c ("xsk: Introduce AF_XDP buffer allocation API")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>

Nice catch!

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

> ---
>  include/net/xsk_buff_pool.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
> index eaa8386dbc63..7a9a23e7a604 100644
> --- a/include/net/xsk_buff_pool.h
> +++ b/include/net/xsk_buff_pool.h
> @@ -147,11 +147,16 @@ static inline bool xp_desc_crosses_non_contig_pg(struct xsk_buff_pool *pool,
>  {
>         bool cross_pg = (addr & (PAGE_SIZE - 1)) + len > PAGE_SIZE;
>
> -       if (pool->dma_pages_cnt && cross_pg) {
> +       if (likely(!cross_pg))
> +               return false;
> +
> +       if (pool->dma_pages_cnt) {
>                 return !(pool->dma_pages[addr >> PAGE_SHIFT] &
>                          XSK_NEXT_PG_CONTIG_MASK);
>         }
> -       return false;
> +
> +       /* skb path */
> +       return addr + len > pool->addrs_cnt;
>  }
>
>  static inline u64 xp_aligned_extract_addr(struct xsk_buff_pool *pool, u64 addr)
>
> base-commit: da5ac772cfe2a03058b0accfac03fad60c46c24d
> --
> 2.29.0
>