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 4A65C4315A; Wed, 22 Jul 2026 15:13:07 +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=1784733188; cv=none; b=ISfbJz0PwuoVlSroN27J6cP9PBSXTu2ackCe81ZflaRzV8X46aNqmfNPBKyjMHAPtKUJO9oc1TWyjXIfDtTbWhXl6lvzvl0TKF94sW1NBzA/m7fKKtfSCKjt9ZI6//S9UGprxYg9TAZN6FRdANHuMSBc4P4GnXXhW49Q8t7owBU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784733188; c=relaxed/simple; bh=m9trUnR4dBBYxmRnQU/Yv79OKE5xa7ig6bqgKmqOEyo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=u+eSs2XXSWUUG02q+E21pe++UJJM90PNWBvBPb/iIAtUfT3EKFM2wWEGmY2ZJgtW0runIX5iUtwciKffGmBt9zGEFiUKpuHIHGOrFA6Uz2LL1StSGehSBTutcigjhw3RDEBjWfvqZQavzx3uJjKo1Go7tGEzuOCmCeSHvx6aJI8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZlsJFjOE; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ZlsJFjOE" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E8FBF1F000E9; Wed, 22 Jul 2026 15:13:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784733186; bh=CdvrZC3DTk1xoDygI32uKfJ+XHbG+XPHXKdsPWJkktw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=ZlsJFjOEqgoYZe3107NxweDyYZlJBBv/VAqKctNP3Bhgab10RBi/veineZ1QhWIaB oj9Mck53nZ1/pA5lebemP4DJ4RVWNW9BRDZ/fQLBFcUECRdrfprojSPBHBQNnJoezN imDGjDHQkGjPQ5rM7ogtRGDX2z2SyIhuAUWZv42Wo3/8iY2pLsvybytPB2vJ1XJleN D5jOAFHhCMrXFBTuLlfOq6hMvDc9KiyIKz+FpUgDfEYNgUJycSmUUhrC517B/XSTyp 6MvahAgC0wnie2lbl9+DQxb5h04rMDDRW4gEoCx07O9Fpv+3FBtA7ZYhjhg/yK2VjQ YyDMnt2S/Ty1g== Date: Wed, 22 Jul 2026 16:13:02 +0100 From: Simon Horman To: Doruk Tan Ozturk Cc: jk@codeconstruct.com.au, matt@codeconstruct.com.au, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH net v2] mctp: serial: handle zero-length frames to prevent rx buffer overflow Message-ID: <20260722151302.GE418547@horms.kernel.org> References: <20260715082021.46315-1-doruk@0sec.ai> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260715082021.46315-1-doruk@0sec.ai> On Wed, Jul 15, 2026 at 10:20:21AM +0200, Doruk Tan Ozturk wrote: > 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