Bluetooth: RFCOMM: Reject short EA=0 frames in rfcomm_recv_frame()

Overview

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: RFCOMM: Reject short EA=0 frames in rfcomm_recv_frame()

While rfcomm_recv_frame() verifies that skb->len is at least sizeof(*hdr) + 1 (4 bytes: 3-byte header + 1-byte FCS), an RFCOMM frame with an extended 2-byte length field (!__test_ea(hdr->len)) has a 4-byte header plus a 1-byte FCS (5 bytes minimum, sizeof(*hdr) + 2).

When a 4-byte RFCOMM frame with EA == 0 arrives:

1. The initial skb->len < sizeof(*hdr) + 1 check passes (4 < 4 is false).
2. Trimming the FCS byte decrements skb->len to 3.
3. If __check_fcs() succeeds, skb_pull(skb, 4) fails (4 > 3) and returns NULL without advancing skb->data.
4. Because the return value of skb_pull() is ignored, the un-pulled 3-byte struct rfcomm_hdr remains at skb->data and is either queued as application payload via rfcomm_recv_data() or parsed as a multiplexer control command via rfcomm_recv_mcc() on DLCI 0.

Fix this by extending the length check in rfcomm_recv_frame() to also require skb->len >= sizeof(*hdr) + 2 when !__test_ea(hdr->len).

Affected

• Linux >= fb40eda15122f6b780228639c1ead6820340ff54 < 76876fd26569302d26d91cbe8b75fa8e66cf8969
• Linux >= 3829af5fab5a22405bf1e7d69068c2ec0fce46ca < 97711b964877242933484989224a3c4c5d88a5ee
• Linux >= b161eacd7fa4a2d765724b5504ac6cc388e48f80 < 7a1781a2f3a9667b3065d402d1f11986a2363e08
• Linux >= 30d1d9f3495463f7c026e4c553127f79b486fcd6 < c25587fa26761e623198b4f47151fe79d09ccde6
• Linux >= 67dc3b40fae71b6b11c71e7ff69bac0758818c05 < 71ccacde407b7d8f0ef48974a0d2524602fd5c44
• Linux >= bbc310caa2b6bf5fe42896ba27da0fbc12e4ac51 < be00deee60044ffa820f9436415caeb665b2cda0
• Linux >= b230e5bf501c5edaf2eb0991cb862ac142031d4b < 9240fc1f2cde5331c61608da21b00394602ca762
• Linux >= b230e5bf501c5edaf2eb0991cb862ac142031d4b < 6d91041bb38b97e2feb625123cc0529d7b83a0e1
• Linux >= 6.12.111 < 6.12.112
• Linux >= 6.18.53 < 6.18.55
• Linux 7.2

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.