From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=50977 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OZNnZ-0003TH-Rz for qemu-devel@nongnu.org; Thu, 15 Jul 2010 08:44:26 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OZNnU-00032x-NW for qemu-devel@nongnu.org; Thu, 15 Jul 2010 08:44:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:21772) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OZNnU-00032e-Gr for qemu-devel@nongnu.org; Thu, 15 Jul 2010 08:44:16 -0400 Message-ID: <4C3F0296.4050300@redhat.com> Date: Thu, 15 Jul 2010 14:44:06 +0200 From: Kevin Wolf MIME-Version: 1.0 References: <1279130069-5331-1-git-send-email-aliguori@us.ibm.com> <4C3EC317.3020706@redhat.com> <4C3EFF05.9010100@linux.vnet.ibm.com> In-Reply-To: <4C3EFF05.9010100@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-15 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: Anthony Liguori Cc: qemu-devel@nongnu.org, Stefan Hajnoczi Am 15.07.2010 14:28, schrieb Anthony Liguori: > 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. We rely on that anyway, we'd overflow iov_len otherwise. Maybe adding an assert there wouldn't hurt. But I'm fine either way. Kevin