Re: [Qemu-devel] [PATCH] Make default invocation of block drivers safer (v3)

[Anthony PERARD <anthony.perard@citrix.com>]

Anthony Liguori wrote:
> 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 <aliguori@us.ibm.com>
> ---
> v2 -> v3
>  - add an assert to ensure the first iovec element is at least 512 bytes
> v1 -> v2
>  - be more paranoid about empty iovecs
> ---
>  block.c     |    4 ++
>  block/raw.c |  130 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  block_int.h |    1 +
>  3 files changed, 135 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) {
> +            assert(qiov->iov[i].iov_len >= 512);
> +            first_buf_index = i;
> +            break;
> +        }
> +    }

Hi,

I have try to do an installation of Windows XP SP2, with qemu fd2f659,
and the Assertion failed when windows begin to format the disk.

The command line and the error message:
$ i386-softmmu/qemu -hda vm.img -cdrom winxpsp2.iso -boot dc
qemu: qemu/block/raw.c:130: raw_aio_writev: Assertion `qiov->iov[i].iov_len >= 512' failed.

And here, a little more information about the iov:
(gdb) p *qiov
$2 = {iov = 0x9106010, niov = 2, nalloc = 2, size = 512}
(gdb) p qiov->iov[0]
$3 = {iov_base = 0xaff3ce90, iov_len = 368}
(gdb) p qiov->iov[1]
$4 = {iov_base = 0xaff3f000, iov_len = 144}

Without the assert, the install work fine.

Regards,

-- 
Anthony PERARD