Re: [Qemu-devel] [oss-security] Re: [PATCH 1/3] virtio-pci: properly validate address before accessing config

[Kurt Seifried]

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On 04/28/2013 05:29 AM, Petr Matousek wrote:
> On Sat, Apr 27, 2013 at 01:13:16PM +0800, Jason Wang wrote:
>> On 04/26/2013 10:27 PM, Petr Matousek wrote:
>>> On Fri, Apr 26, 2013 at 04:34:02PM +0800, Jason Wang wrote:
>>>> There are several several issues in the current checking:
>>>> 
>>>> - The check was based on the minus of unsigned values which
>>>> can overflow - It was done after .{set|get}_config() which
>>>> can lead crash when config_len is zero since vdev->config is
>>>> NULL
>>>> 
>>>> Fix this by:
>>>> 
>>>> - Validate the address in virtio_pci_config_{read|write}()
>>>> before .{set|get}_config - Use addition instead minus to do
>>>> the validation
>>>> 
>>>> Cc: Michael S. Tsirkin <address@hidden> Cc: Petr Matousek
>>>> <address@hidden> Signed-off-by: Jason Wang
>>>> <address@hidden> --- hw/virtio/virtio-pci.c |    9
>>>> +++++++++ hw/virtio/virtio.c     |   18 ------------------ 2
>>>> files changed, 9 insertions(+), 18 deletions(-)
>>>> 
>>>> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c 
>>>> index a1f15a8..7f6c7d1 100644 --- a/hw/virtio/virtio-pci.c 
>>>> +++ b/hw/virtio/virtio-pci.c @@ -400,6 +400,10 @@ static
>>>> uint64_t virtio_pci_config_read(void *opaque, hwaddr addr, } 
>>>> addr -= config;
>>>> 
>>>> +    if (addr + size > proxy->vdev->config_len) { +
>>>> return (uint32_t)-1; +    } +
>>> What is the range of values addr can be? I guess it's not
>>> arbitrary and not fully in guests hands. Can it be higher than
>>> corresponding pci config space size?
>> 
>> Not fully in guests hands. It depends on size the config size. 
>> Unfortunately, qemu will roundup the size to power of 2 in 
>> virtio_pci_device_plugged():
>> 
>> size = VIRTIO_PCI_REGION_SIZE(&proxy->pci_dev) +
>> virtio_bus_get_vdev_config_len(bus);
>> 
>> if (size & (size - 1)) { size = 1 << qemu_fls(size); }
>> 
>> So, for virtio-rng, though its region size is 20, it will be
>> rounded up to 32, which left guest the possibility to access
>> beyond the config space. So some check is needs in
>> virito_pci_config_read().
> 
> Ok, in that case it would make sense to document the preconditions
> that assures that addr + size won't overflow. Or add the values in
> a safe way (check that the sum is not less than one of the
> addend).
> 
>>> IOW, can guest touch anything interesting or will all accesses
>>> end in the first page in the qemu address space, considering
>>> vdev->config being NULL?
>>> 
>> 
>> There's another theoretical issue as pointed by Anthony, see 
>> virtio_config_writew():
>> 
>> void virtio_config_writew(VirtIODevice *vdev, uint32_t addr,
>> uint32_t data) { uint16_t val = data;
>> 
>> if (addr > (vdev->config_len - sizeof(val))) return;
>> 
>> stw_p(vdev->config + addr, val);
>> 
>> if (vdev->set_config) vdev->set_config(vdev, vdev->config); }
>> 
>> If there's a device whose config_len is 1, the check will fail
>> and we can access some other location.
>> 
>> But since virtio-rng has zero config length and addr here should
>> be less than 12, and all other device's config length is all
>> greater than 4. Only first page could be access here.
> 
> So the only practical attack (virtio-rng device that has config
> length 0) can only end in the first page of qemu address space
> which is on any not-so-much recent kernel protected by
> mmap_min_addr and will result in qemu process crash. Access to pci
> config space is privileged operation, so root user in the guest can
> crash the guest (something that root can do anyways).
> 
> Don't get me wrong, we still need the fix to avoid any potential
> issues in the future, but I'm leaning towards not treating this
> issue as a security (CVE) one due to the lack of practical
> exploitability.
> 
> @Kurt -- do we assign CVE identifiers to issues that rely on an
> option (or lack of) that when set in a way that would allow the
> issue in question to be exploited is known to be insecure?
> 
> References: 
> https://lists.gnu.org/archive/html/qemu-devel/2013-04/msg05013.html
>
> 
https://bugzilla.redhat.com/show_bug.cgi?id=957155
> 
> The option is mmap_min_addr which assures that no mapping can be
> present at the beginning of the address space and all accesses will
> result in sigsegv. Default setting of mmap_min_addr is enough to
> avoid this issue from having security consequences. Disabling
> mmap_min_addr (setting to 0) means that some if not all of the
> "kernel NULL pointer dereferences" out there could be used for
> privilege escalation.
> 
> Thanks,
> 

Ok so this does NOT affect Linux systems using mmap_min_addr with a
sane default value (e.g. larger than 0). However more than a few
systems have shipped with mmap_min_addr set to 0, not picking on
Debian, just the first result I found:

http://wiki.debian.org/mmap_min_addr

"In Debian 5.0.0 through 5.0.3 inclusive, the 2.6.26 kernel is shipped
with a default mmap_min_addr of '0'."

Hopefully everyone has upgraded to 5.0.4 =).

So there is a realistic potential for systems to be affected (e.g. if
this had been fixed 10 years ago I'd have probably not given a CVE,
but 3 years is not a super long time).

Please use CVE-2013-2016 for virtio-pci: properly validate address
before accessing config.



- -- 
Kurt Seifried Red Hat Security Response Team (SRT)
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