From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38905) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aZBbx-0004Wm-M6 for qemu-devel@nongnu.org; Fri, 26 Feb 2016 01:10:46 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aZBbt-0000c3-MI for qemu-devel@nongnu.org; Fri, 26 Feb 2016 01:10:45 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39148) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aZBbt-0000br-GE for qemu-devel@nongnu.org; Fri, 26 Feb 2016 01:10:41 -0500 References: <1456243707-29345-1-git-send-email-ppandit@redhat.com> <1456243707-29345-2-git-send-email-ppandit@redhat.com> From: Jason Wang Message-ID: <56CFEC59.8040503@redhat.com> Date: Fri, 26 Feb 2016 14:10:33 +0800 MIME-Version: 1.0 In-Reply-To: <1456243707-29345-2-git-send-email-ppandit@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2 1/2] net: check packet payload length List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: P J P , Qemu Developers Cc: Liu Ling , Prasad J Pandit , Markus Armbruster On 02/24/2016 12:08 AM, P J P wrote: > From: Prasad J Pandit > > While computing IP checksum, 'net_checksum_calculate' reads > payload length from the packet. It could exceed the given 'data' > buffer size. Add a check to avoid it. > > Reported-by: Liu Ling > Signed-off-by: Prasad J Pandit > --- > net/checksum.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > Update as per review: > -> https://lists.gnu.org/archive/html/qemu-devel/2016-02/msg04062.html > > diff --git a/net/checksum.c b/net/checksum.c > index 14c0855..bd89083 100644 > --- a/net/checksum.c > +++ b/net/checksum.c > @@ -59,6 +59,11 @@ void net_checksum_calculate(uint8_t *data, int length) > int hlen, plen, proto, csum_offset; > uint16_t csum; > I must say this is still far from perfect, since it has too assumptions . But I agree we can fix OOB first. > + /* Ensure data has complete L2 & L3 headers. */ > + if (length < 14 + 20) { > + return; > + } > + > if ((data[14] & 0xf0) != 0x40) > return; /* not IPv4 */ > hlen = (data[14] & 0x0f) * 4; > @@ -76,8 +81,9 @@ void net_checksum_calculate(uint8_t *data, int length) > return; > } > > - if (plen < csum_offset+2) > - return; > + if (plen < csum_offset + 2 || plen + hlen >= length) { > + return; > + } Should we count mac header here? Did "plen + hlen >= length" imply "14 + hlen + csum_offset + 1" < length? Looks not. Consider a TCP packet can report evil plen (e.g 20) but just have 10 bytes payload in fact. In this case: hlen = 20, plen = 20, csum_offset = 16, length = 44 which can pass all the above tests, but 14 + hlen + csum_offset = 50 which is greater than length. > > data[14+hlen+csum_offset] = 0; > data[14+hlen+csum_offset+1] = 0;