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Tue, 19 May 2026 00:26:07 -0700 (PDT) From: Muhammad Bilal To: linux-bluetooth@vger.kernel.org Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, marcel@holtmann.org, luiz.dentz@gmail.com, kees@kernel.org, kuba@kernel.org, stable@vger.kernel.org, Muhammad Bilal Subject: [PATCH v2] Bluetooth: RFCOMM: add minimum length check in rfcomm_recv_frame Date: Tue, 19 May 2026 03:25:47 -0400 Message-ID: <20260519072547.33974-1-meatuni001@gmail.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260519042017.29564-1-meatuni001@gmail.com> References: <20260519042017.29564-1-meatuni001@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" rfcomm_recv_frame() casts skb->data to struct rfcomm_hdr * and immediately dereferences hdr->addr and hdr->ctrl without first validating that skb->len is large enough to hold the header. A remote device can send a crafted short RFCOMM frame over L2CAP to trigger an out-of-bounds read before any session state is checked. The FCS trimming code that follows compounds the problem: skb->len--; skb->tail--; If skb->len is already zero the decrement wraps to UINT_MAX, causing skb_tail_pointer() to return a pointer far outside the skb and producing a second out-of-bounds read when the FCS byte is consumed. Add a minimum length check before the header pointer is assigned. A well-formed RFCOMM frame requires at least addr(1) + ctrl(1) + len(1) + fcs(1) =3D sizeof(struct rfcomm_hdr) + 1 bytes. This single guard prevents both the header out-of-bounds read and the skb->len integer underflow. Note: SeungJu Cheon posted a related patch that adds equivalent length checks inside the individual MCC sub-handlers (rfcomm_recv_pn, rfcomm_recv_rpn, rfcomm_recv_rls, rfcomm_recv_msc, rfcomm_recv_mcc). That fix and this one are complementary and independent; neither subsumes the other. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org Signed-off-by: Muhammad Bilal --- net/bluetooth/rfcomm/core.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c index d11bd5337..6b300237c 100644 --- a/net/bluetooth/rfcomm/core.c +++ b/net/bluetooth/rfcomm/core.c @@ -1741,7 +1741,7 @@ static int rfcomm_recv_data(struct rfcomm_session *s,= u8 dlci, int pf, struct sk static struct rfcomm_session *rfcomm_recv_frame(struct rfcomm_session *s, struct sk_buff *skb) { - struct rfcomm_hdr *hdr =3D (void *) skb->data; + struct rfcomm_hdr *hdr; u8 type, dlci, fcs; =20 if (!s) { @@ -1750,10 +1750,17 @@ static struct rfcomm_session *rfcomm_recv_frame(str= uct rfcomm_session *s, return s; } =20 + /* Minimum valid frame: addr(1) + ctrl(1) + len(1) + fcs(1) */ + if (skb->len < sizeof(*hdr) + 1) { + kfree_skb(skb); + return s; + } + + hdr =3D (void *) skb->data; dlci =3D __get_dlci(hdr->addr); type =3D __get_type(hdr->ctrl); =20 - /* Trim FCS */ + /* Trim FCS - safe: skb->len >=3D sizeof(*hdr) + 1 >=3D 1 */ skb->len--; skb->tail--; fcs =3D *(u8 *)skb_tail_pointer(skb); =20 --=20 2.54.0