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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 8 Jun 2022 08:33:56 -0700
Message-ID: <CANn89iJ2gf4JfU8KZUYFSA8KgS-gEjhBZtX9WvUmWv2c8kPkJQ@mail.gmail.com>
Subject: Re: [PATCH] Bluetooth: RFCOMM: Use skb_trim to trim checksum
To:     Soenke Huster <soenke.huster@eknoes.de>
Cc:     Marcel Holtmann <marcel@holtmann.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        linux-bluetooth@vger.kernel.org, netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Jun 8, 2022 at 6:51 AM Soenke Huster <soenke.huster@eknoes.de> wrote:
>
> Use the skb helper instead of direct manipulation. This fixes the
> following page fault, when connecting my Android phone:
>
>     BUG: unable to handle page fault for address: ffffed1021de29ff
>     #PF: supervisor read access in kernel mode
>     #PF: error_code(0x0000) - not-present page
>     RIP: 0010:rfcomm_run+0x831/0x4040 (net/bluetooth/rfcomm/core.c:1751)
>
> Signed-off-by: Soenke Huster <soenke.huster@eknoes.de>
> ---
>  net/bluetooth/rfcomm/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
> index 7324764384b6..7360e905d045 100644
> --- a/net/bluetooth/rfcomm/core.c
> +++ b/net/bluetooth/rfcomm/core.c
> @@ -1747,8 +1747,8 @@ static struct rfcomm_session *rfcomm_recv_frame(struct rfcomm_session *s,
>         type = __get_type(hdr->ctrl);
>
>         /* Trim FCS */
> -       skb->len--; skb->tail--;
> -       fcs = *(u8 *)skb_tail_pointer(skb);
> +       skb_trim(skb, skb->len - 1);
> +       fcs = *(skb->data + skb->len);
>

Hmmm... I do not see any difference before/after in term of memory
dereference to get fcs.

I think you should give more details on how exactly the bug triggers.