[Qemu-devel] Re: [PATCH] Make default invocation of block drivers safer (v2)

[Kevin Wolf]

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 <address@hidden>
> ---
> 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.

Kevin