From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=58674 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OZNYg-0002ni-OE for qemu-devel@nongnu.org; Thu, 15 Jul 2010 08:29:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OZNYb-0008SB-Lk for qemu-devel@nongnu.org; Thu, 15 Jul 2010 08:28:58 -0400 Received: from e8.ny.us.ibm.com ([32.97.182.138]:34271) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OZNYb-0008RU-JQ for qemu-devel@nongnu.org; Thu, 15 Jul 2010 08:28:53 -0400 Received: from d01relay07.pok.ibm.com (d01relay07.pok.ibm.com [9.56.227.147]) by e8.ny.us.ibm.com (8.14.4/8.13.1) with ESMTP id o6F8GL1r011082 for ; Thu, 15 Jul 2010 04:16:21 -0400 Received: from d01av02.pok.ibm.com (d01av02.pok.ibm.com [9.56.224.216]) by d01relay07.pok.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id o6FCSm0d1921190 for ; Thu, 15 Jul 2010 08:28:48 -0400 Received: from d01av02.pok.ibm.com (loopback [127.0.0.1]) by d01av02.pok.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id o6FCSlhc022953 for ; Thu, 15 Jul 2010 09:28:48 -0300 Message-ID: <4C3EFF05.9010100@linux.vnet.ibm.com> Date: Thu, 15 Jul 2010 07:28:53 -0500 From: Anthony Liguori MIME-Version: 1.0 References: <1279130069-5331-1-git-send-email-aliguori@us.ibm.com> <4C3EC317.3020706@redhat.com> In-Reply-To: <4C3EC317.3020706@redhat.com> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: [PATCH] Make default invocation of block drivers safer (v2) List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: qemu-devel@nongnu.org, Stefan Hajnoczi On 07/15/2010 03:13 AM, Kevin Wolf wrote: > Am 14.07.2010 19:54, schrieb Anthony Liguori: > >> CVE-2008-2004 described a vulnerability in QEMU whereas a malicious user could >> trick the block probing code into accessing arbitrary files in a guest. To >> mitigate this, we added an explicit format parameter to -drive which disabling >> block probing. >> >> Fast forward to today, and the vast majority of users do not use this parameter. >> libvirt does not use this by default nor does virt-manager. >> >> Most users want block probing so we should try to make it safer. >> >> This patch adds some logic to the raw device which attempts to detect a write >> operation to the beginning of a raw device. If the first 4 bytes happen to >> match an image file that has a backing file that we support, it scrubs the >> signature to all zeros. If a user specifies an explicit format parameter, this >> behavior is disabled. >> >> I contend that while a legitimate guest could write such a signature to the >> header, we would behave incorrectly anyway upon the next invocation of QEMU. >> This simply changes the incorrect behavior to not involve a security >> vulnerability. >> >> I've tested this pretty extensively both in the positive and negative case. I'm >> not 100% confident in the block layer's ability to deal with zero sized writes >> particularly with respect to the aio functions so some additional eyes would be >> appreciated. >> >> Even in the case of a single sector write, we have to make sure to invoked the >> completion from a bottom half so just removing the zero sized write is not an >> option. >> >> Signed-off-by: Anthony Liguori >> --- >> v1 -> v2 >> - be more paranoid about empty iovecs >> --- >> block.c | 4 ++ >> block/raw.c | 129 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >> block_int.h | 1 + >> 3 files changed, 134 insertions(+), 0 deletions(-) >> >> > >> static BlockDriverAIOCB *raw_aio_writev(BlockDriverState *bs, >> int64_t sector_num, QEMUIOVector *qiov, int nb_sectors, >> BlockDriverCompletionFunc *cb, void *opaque) >> { >> + const uint8_t *first_buf; >> + int first_buf_index = 0, i; >> + >> + /* This is probably being paranoid, but handle cases of zero size >> + vectors. */ >> + for (i = 0; i< qiov->niov; i++) { >> + if (qiov->iov[i].iov_len) { >> + first_buf_index = i; >> + break; >> + } >> + } >> + >> + first_buf = qiov->iov[first_buf_index].iov_base; >> > It's still not paranoid enough for the case where the magic is spread > over multiple buffers. We should probably have a qemu_iovec_to_buffer() > with limited size so that you can just get 4 bytes into a temporary buffer. > I'm quite confident that iovec buffers are never less than 512 in size. While it could be more paranoid, I don't think the added complexity helps. Regards, Anthony Liguori > Kevin >