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

[Kevin Wolf <kwolf@redhat.com>]

Am 15.07.2010 14:50, 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 <aliguori@us.ibm.com>

Acked-by: Kevin Wolf <kwolf@redhat.com>