From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 16D3344839B; Thu, 30 Jul 2026 15:23:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785425033; cv=none; b=COMf7htsaqsC6tQrpriVB9PDrunVNcITlPQDWOkXFolTxXBXTGcDfy1QEfKnFVjlaurbn5J0f2/g7Z8jwuM5zkuzmlUVNKRFYCWIqeJ+vsopTdQTP8+5sXnviCowyjhUW69ezL605EEAFyzy2TMbKDfLDiSmsCReKkMjm7jJfxY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785425033; c=relaxed/simple; bh=c/JTqDHpxjCO/4e5v0yP/pp9UO1BrHGV0bqZP0IVBY4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=HK3+CtaISdOEHghK6618PwQWgA1jjlsBpXGcnbeNXDP/+gkvnN9GuAmfcGnUo6daJpeJVgp1Ylpi61iZIPpjx4JurPvoRMQ00KRrX3bv6N8YF355/g/8X3OQLCIr9mFxl7zfuub3PtLN3YEiGOBzfPMHQeGbI5AiCa5f+1xPy64= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=RxpicD2o; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="RxpicD2o" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 249981F000E9; Thu, 30 Jul 2026 15:23:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1785425030; bh=WoGjk+dMzE6VPtnErMcsZmEkK/v5X7UYLlOtixGpArk=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=RxpicD2op83suR39apW6jzU81ugEwCLb8zIJ1crnArY28NUL2wcq0pR6LpBB6Wzqi DqEwRTCcuCcEJ44F7pxBtka8hudtg8yU5nTBZY0pPp60uJzJHxDWgHvhtLDFlbeHr9 TLNxof7/N/Ufa00rnC2I9EwncZI46W4J/8IUI/iw= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Jeremy Kerr , Doruk Tan Ozturk , Simon Horman , Paolo Abeni Subject: [PATCH 6.18 558/675] mctp: serial: handle zero-length frames to prevent rx buffer overflow Date: Thu, 30 Jul 2026 16:14:48 +0200 Message-ID: <20260730141456.995841146@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260730141445.110192266@linuxfoundation.org> References: <20260730141445.110192266@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Doruk Tan Ozturk commit 793b9b729f1e8de57be8c8daf1a9838be96cabed upstream. The MCTP serial receive state machine reads a frame length byte in mctp_serial_push_header() case 2 and validates it upper-bound-only: if (c > MCTP_SERIAL_FRAME_MTU) { dev->rxstate = STATE_ERR; } else { dev->rxlen = c; dev->rxpos = 0; dev->rxstate = STATE_DATA; ... } A length of zero passes this check, so rxlen is set to 0 and the state machine advances to STATE_DATA. In mctp_serial_push() STATE_DATA, the incoming byte is stored and rxpos incremented before the terminator is tested: dev->rxbuf[dev->rxpos] = c; dev->rxpos++; dev->rxstate = STATE_DATA; if (dev->rxpos == dev->rxlen) { dev->rxpos = 0; dev->rxstate = STATE_TRAILER; } With rxlen == 0 the "rxpos == rxlen" terminator can never fire (rxpos is already 1 on the first data byte), so subsequent bytes are written past the end of the fixed 74-byte rxbuf, which is the last member of the netdev private area. Every following data byte is an attacker-controlled 1-byte out-of-bounds heap write, and the overflow continues until a frame (0x7e) or escape byte resets the parser -- effectively unbounded. Reaching this requires CAP_NET_ADMIN to attach the N_MCTP line discipline and bring the resulting mctpserialN netdev up, after which the bytes arrive via the tty receive path. Route a zero-length frame straight to STATE_TRAILER instead of STATE_DATA. The trailer/framing bytes are still consumed, and the frame resolves to a zero-length skb that the MCTP core rejects; the parser never enters STATE_DATA with rxlen == 0, so the out-of-bounds write can no longer occur. KASAN, on a frame of 0x7e 0x01 0x00 followed by data bytes (before this change): UBSAN: array-index-out-of-bounds in drivers/net/mctp/mctp-serial.c:370 index 74 is out of range for type 'u8 [74]' BUG: KASAN: slab-out-of-bounds in mctp_serial_tty_receive_buf Write of size 1 at addr ... by task kworker/u16:0 mctp_serial_tty_receive_buf tty_ldisc_receive_buf flush_to_ldisc Allocated by task 152: alloc_netdev_mqs mctp_serial_open v2: route zero-length frames to STATE_TRAILER instead of STATE_ERR so the trailer/framing bytes are still consumed (Jeremy Kerr). Found by 0sec automated security-research tooling (https://0sec.ai). Fixes: a0c2ccd9b5ad ("mctp: Add MCTP-over-serial transport binding") Cc: stable@vger.kernel.org Suggested-by: Jeremy Kerr Assisted-by: 0sec:multi-model Signed-off-by: Doruk Tan Ozturk Reviewed-by: Simon Horman Link: https://patch.msgid.link/20260715082021.46315-1-doruk@0sec.ai Signed-off-by: Paolo Abeni Signed-off-by: Greg Kroah-Hartman --- drivers/net/mctp/mctp-serial.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/drivers/net/mctp/mctp-serial.c +++ b/drivers/net/mctp/mctp-serial.c @@ -318,7 +318,7 @@ static void mctp_serial_push_header(stru } else { dev->rxlen = c; dev->rxpos = 0; - dev->rxstate = STATE_DATA; + dev->rxstate = c > 0 ? STATE_DATA : STATE_TRAILER; dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c); } break;