Re: [RFC PATCH 0/7] Untrusted device support for virtio

From: Jason Wang
Date: Sun Jun 06 2021 - 22:47:17 EST


在 2021/6/4 下午11:17, Konrad Rzeszutek Wilk 写道:
On 4/29/21 12:16 AM, Jason Wang wrote:

在 2021/4/29 上午5:06, Konrad Rzeszutek Wilk 写道:
On Wed, Apr 21, 2021 at 11:21:10AM +0800, Jason Wang wrote:
Hi All:

Sometimes, the driver doesn't trust the device. This is usually
happens for the encrtpyed VM or VDUSE[1]. In both cases, technology
like swiotlb is used to prevent the poking/mangling of memory from the
device. But this is not sufficient since current virtio driver may
trust what is stored in the descriptor table (coherent mapping) for
performing the DMA operations like unmap and bounce so the device may
choose to utilize the behaviour of swiotlb to perform attacks[2].
We fixed it in the SWIOTLB. That is it saves the expected length
of the DMA operation. See

commit daf9514fd5eb098d7d6f3a1247cb8cc48fc94155
Author: Martin Radev <martin.b.radev@xxxxxxxxx>
Date:   Tue Jan 12 16:07:29 2021 +0100

     swiotlb: Validate bounce size in the sync/unmap path
     The size of the buffer being bounced is not checked if it happens
     to be larger than the size of the mapped buffer. Because the size
     can be controlled by a device, as it's the case with virtio devices,
     this can lead to memory corruption.


Good to know this, but this series tries to protect at different level. And I believe such protection needs to be done at both levels.


My apologies for taking so long to respond, somehow this disappeared in one of the folders.


No problem.



For double insurance, to protect from a malicous device, when DMA API
is used for the device, this series store and use the descriptor
metadata in an auxiliay structure which can not be accessed via
swiotlb instead of the ones in the descriptor table. Actually, we've
Sorry for being dense here, but how wold SWIOTLB be utilized for
this attack?


So we still behaviors that is triggered by device that is not trusted. Such behavior is what the series tries to avoid. We've learnt a lot of lessons to eliminate the potential attacks via this. And it would be too late to fix if we found another issue of SWIOTLB.

Proving "the unexpected device triggered behavior is safe" is very hard (or even impossible) than "eliminating the unexpected device triggered behavior totally".

E.g I wonder whether something like this can happen: Consider the DMA direction of unmap is under the control of device. The device can cheat the SWIOTLB by changing the flag to modify the device read only buffer.

<blinks> Why would you want to expose that to the device? And wouldn't that be specific to Linux devices - because surely Windows DMA APIs are different and this 'flag' seems very Linux-kernel specific?


Just to make sure we are in the same page. The "flag" I actually mean the virtio descriptor flag which could be modified by the device. And driver deduce the DMA API flag from the descriptor flag.



If yes, it is really safe?

Well no? But neither is rm -Rf / but we still allow folks to do that.

The above patch only log the bounce size but it doesn't log the flag.

It logs and panics the system.


Good to know that.



Even if it logs the flag, SWIOTLB still doesn't know how each buffer is used and when it's the appropriate(safe) time to unmap the buffer, only the driver that is using the SWIOTLB know them.

Fair enough. Is the intent to do the same thing for all the other drivers that could be running in an encrypted guest and would require SWIOTLB.

Like legacy devices that KVM can expose (floppy driver?, SVGA driver)?


My understanding is that we shouldn't enable the legacy devices at all in this case.

Note that virtio has been extended to various types of devices (we can boot qemu without PCI and legacy devices (e.g the micro VM))

- virtio input
- virtio gpu
- virtio sound
...

I'm not sure whether we need floppy, but it's not hard to have a virtio-floppy if necessary

So it would be sufficient for us to audit/harden the virtio drivers.




So I think we need to consolidate on both layers instead of solely depending on the SWIOTLB.

Please make sure that this explanation is in part of the cover letter
or in the commit/Kconfig.


I will do that if the series needs a respin.



Also, are you aware of the patchset than Andi been working on that tries to make the DMA code to have extra bells and whistles for this purpose?


Yes, but as described above they are not duplicated. Protection at both levels would be optimal.

Another note is that this series is not only for DMA/swiotlb stuffs, it eliminate all the possible attacks via the descriptor ring.

(One example is the attack via descriptor.next)

Thanks



Thank you.
Thanks



almost achieved that through packed virtqueue and we just need to fix
a corner case of handling mapping errors. For split virtqueue we just
follow what's done in the packed.

Note that we don't duplicate descriptor medata for indirect
descriptors since it uses stream mapping which is read only so it's
safe if the metadata of non-indirect descriptors are correct.

The behaivor for non DMA API is kept for minimizing the performance
impact.

Slightly tested with packed on/off, iommu on/of, swiotlb force/off in
the guest.

Please review.

[1] https://lore.kernel.org/netdev/fab615ce-5e13-a3b3-3715-a4203b4ab010@xxxxxxxxxx/T/
[2] https://yhbt.net/lore/all/c3629a27-3590-1d9f-211b-c0b7be152b32@xxxxxxxxxx/T/#mc6b6e2343cbeffca68ca7a97e0f473aaa871c95b

Jason Wang (7):
   virtio-ring: maintain next in extra state for packed virtqueue
   virtio_ring: rename vring_desc_extra_packed
   virtio-ring: factor out desc_extra allocation
   virtio_ring: secure handling of mapping errors
   virtio_ring: introduce virtqueue_desc_add_split()
   virtio: use err label in __vring_new_virtqueue()
   virtio-ring: store DMA metadata in desc_extra for split virtqueue

  drivers/virtio/virtio_ring.c | 189 ++++++++++++++++++++++++++---------
  1 file changed, 141 insertions(+), 48 deletions(-)

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2.25.1