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Date: Mon, 29 Apr 2013 10:51:44 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Kurt Seifried <kseifried@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>, Kurt Seifried <kseifrie@...hat.com>,
        aliguori@...ibm.com, qemu-devel@...gnu.org,
        oss-security@...ts.openwall.com
Subject: Re: Re: [PATCH 1/3] virtio-pci: properly validate
 address before accessing config

On Mon, Apr 29, 2013 at 01:34:44AM -0600, Kurt Seifried wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> 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 <mst@...hat.com> Cc: Petr Matousek
> >>>> <pmatouse@...hat.com> Signed-off-by: Jason Wang
> >>>> <jasowang@...hat.com> --- 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.
> 

Please note this only affects virtio-rng which appeared in qemu v1.3.0 and on,
released on Mon Dec 3 2012. So you'd need a distro with an old kernel
that has an updated qemu or qemu-kvm (or backported the bug in virtio-rng).

-- 
MST

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