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From:   Soenke Huster <soenke.huster@eknoes.de>
To:     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>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>
Cc:     Soenke Huster <soenke.huster@eknoes.de>,
        linux-bluetooth@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH v2] Bluetooth: RFCOMM: Use skb_trim to trim checksum
Date:   Fri, 10 Jun 2022 13:07:49 +0200
Message-Id: <20220610110749.110881-1-soenke.huster@eknoes.de>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

As skb->tail might be zero, it can underflow. This leads to a page
fault: skb_tail_pointer simply adds skb->tail (which is now MAX_UINT)
to skb->head.

    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)

By using skb_trim instead of the direct manipulation, skb->tail
is reset. Thus, the correct pointer to the checksum is used.

Signed-off-by: Soenke Huster <soenke.huster@eknoes.de>
---
v2: Clarified how the bug triggers, minimize code change

 net/bluetooth/rfcomm/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
index 7324764384b6..443b55edb3ab 100644
--- a/net/bluetooth/rfcomm/core.c
+++ b/net/bluetooth/rfcomm/core.c
@@ -1747,7 +1747,7 @@ static struct rfcomm_session *rfcomm_recv_frame(struct rfcomm_session *s,
 	type = __get_type(hdr->ctrl);
 
 	/* Trim FCS */
-	skb->len--; skb->tail--;
+	skb_trim(skb, skb->len - 1);
 	fcs = *(u8 *)skb_tail_pointer(skb);
 
 	if (__check_fcs(skb->data, type, fcs)) {
-- 
2.36.1