Re: [virtio-dev] [PATCH RFC 3/3] rng: leak detection support

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 13, 2023 at 09:49:40AM +0200, Babis Chalios wrote:
> 
> 
> On 28/9/23 20:16, Babis Chalios wrote:
> > 
> > 
> > On 27/9/23 23:47, Michael S. Tsirkin wrote:
> > > On Wed, Sep 27, 2023 at 12:43:20PM +0200, Babis Chalios wrote:
> > > > On 22/9/23 18:01, Michael S. Tsirkin wrote:
> > > > > On Fri, Sep 22, 2023 at 05:40:50PM +0200, Babis Chalios wrote:
> > > > > > On 22/9/23 17:06, Michael S. Tsirkin wrote:
> > > > > > > On Tue, Sep 19, 2023 at 12:11:37PM +0200, Babis Chalios wrote:
> > > > > > > > On 19/9/23 12:01, Michael S. Tsirkin wrote:
> > > > > > > > > On Tue, Sep 19, 2023 at 09:32:08AM +0200, Babis Chalios wrote:
> > > > > > > > > > Resending to fix e-mail formatting issues (sorry for the spam)
> > > > > > > > > > 
> > > > > > > > > > On 18/9/23 18:30, Babis Chalios wrote:
> > > > > > > > > > > > > > > Yes, that's what the driver does now in the RFC patch.
> > > > > > > > > > > > > > > However, this just
> > > > > > > > > > > > > > > decreases
> > > > > > > > > > > > > > > the race window, it doesn't eliminate it. If a third
> > > > > > > > > > > > > > > leak event happens it
> > > > > > > > > > > > > > > might not
> > > > > > > > > > > > > > > find any buffers to use:
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > 1. available buffers to queue 1-X
> > > > > > > > > > > > > > > 2. available buffers to queue X
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > 3. poll queue X
> > > > > > > > > > > > > > > 4. used buffers in queue X       <- leak event 1 will
> > > > > > > > > > > > > > > use buffers in X
> > > > > > > > > > > > > > > 5. avail buffers in queue X
> > > > > > > > > > > > > > > 6. poll queue 1-X                <- leak event 2 will
> > > > > > > > > > > > > > > use buffers in 1-X
> > > > > > > > > > > > > > > 7. used buffers in queue 1-X
> > > > > > > > > > > > > > > 8. avail buffers in queue 1-X
> > > > > > > > > > > > > > > <- leak event 3 (it
> > > > > > > > > > > > > > > needs buffers in X, race with step 5)
> > > > > > > > > > > > > > > 9. goto 3
> > > > > > > > > > > > > > I don't get it. we added buffers in step 5.
> > > > > > > > > > > > > What if the leak event 3 arrives before step 5 had time to
> > > > > > > > > > > > > actually add the
> > > > > > > > > > > > > buffers in X and make
> > > > > > > > > > > > > them visible to the device?
> > > > > > > > > > > > Then it will see a single event
> > > > > > > > > > > > in 1-X instead of two events.  A
> > > > > > > > > > > > leak is
> > > > > > > > > > > > a leak though, I don't see does it matter how many triggered.
> > > > > > > > > > > > 
> > > > > > > > > > So the scenario I have in mind is the following:
> > > > > > > > > > 
> > > > > > > > > > (Epoch here is terminology that I used
> > > > > > > > > > in the Linux RFC. It is a value
> > > > > > > > > > maintained by random.c
> > > > > > > > > > that changes every time a leak event happens).
> > > > > > > > > > 
> > > > > > > > > > 1. add buffers to 1-X
> > > > > > > > > > 2. add buffers to X
> > > > > > > > > > 3. poll queue X
> > > > > > > > > > 4. vcpu 0: get getrandom() entropy and cache epoch value
> > > > > > > > > > 5. Device: First snapshot, uses buffers in X
> > > > > > > > > > 6. vcpu 1: sees used buffers
> > > > > > > > > > 7. Device: Second snapshot, uses buffers in 1-X
> > > > > > > > > > 8. vcpu 0: getrandom() observes new  epoch value & caches it
> > > > > > > > > > 9. Device: Third snapshot, no buffers in
> > > > > > > > > > either queue, (vcpu 1 from step 6
> > > > > > > > > > has not yet finished adding new buffers).
> > > > > > > > > > 10. vcpu 1 adds new buffer in X
> > > > > > > > > > 11. vcpu 0: getrandom() will not see new
> > > > > > > > > > epoch and gets stale entropy.
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > > In this succession of events, when the
> > > > > > > > > > third snapshot will happen, the
> > > > > > > > > > device won't find
> > > > > > > > > > any buffers in either queue, so it won't
> > > > > > > > > > increase the RNG epoch value. So,
> > > > > > > > > > any entropy
> > > > > > > > > > gathered after step 8 will be the same
> > > > > > > > > > across all snapshots. Am I missing
> > > > > > > > > > something?
> > > > > > > > > > 
> > > > > > > > > > Cheers,
> > > > > > > > > > Babis
> > > > > > > > > > 
> > > > > > > > > Yes but notice how this is followed by:
> > > > > > > > > 
> > > > > > > > > 12. vcpu 1: sees used buffers in 1-X
> > > > > > > > > 
> > > > > > > > > Driver can notify getrandom I guess?
> > > > > > > > It could, but then we have the exact race
> > > > > > > > condition that VMGENID had,
> > > > > > > > userspace has already consumed stale entropy and there's nothing we
> > > > > > > > can do about that.
> > > > > > > > 
> > > > > > > > Although this is indeed a corner case, it feels
> > > > > > > > like it beats the purpose
> > > > > > > > of having the hardware update directly userspace
> > > > > > > > (via copy on leak).
> > > > > > > > 
> > > > > > > > How do you feel about the proposal a couple of
> > > > > > > > emails back? It looks to
> > > > > > > > me that it avoids completely the race condition.
> > > > > > > > 
> > > > > > > > Cheers,
> > > > > > > > Babis
> > > > > > > It does. The problem of course is that this means that e.g.
> > > > > > > taking a snapshot of a guest that is stuck won't work well.
> > > > > > That is true, but does it matter? The intention of the proposal
> > > > > > is that if it is not safe to take snapshots (i.e. no buffers in the
> > > > > > queue) don't take snapshots.
> > > > > > 
> > > > > > > I have been thinking of adding MAP/UNMAP descriptors for
> > > > > > > a while now. Thus it will be possible to modify
> > > > > > > userspace memory without consuming buffers.
> > > > > > > Would something like this solve the problem?
> > > > > > I am not familiar with MAP/UNMAP descriptors. Is there
> > > > > > a link where I can read about them?
> > > > > > 
> > > > > > Cheers,
> > > > > > Babis
> > > > > Heh no I just came up with the name. Will write up in a couple
> > > > > of days, but the idea is that driver does get_user_pages,
> > > > > adds buffer to queue, and device will remember the address
> > > > > and change that memory on a snapshot. If there are buffers
> > > > > in the queue it will also use these to tell driver,
> > > > > but if there are no buffers then it won't.
> > > > That sounds like a nice mechanism. However in our case the page
> > > > holding the counter that gets increased by the hardware is a kernel
> > > > page.
> > > > 
> > > > The reason for that is that things other than us (virtio-rng) might
> > > > want to notify for leak events. For example, I think that Jason
> > > > intended to use this mechanism to periodically notify user-space
> > > > PRNGs that they need to reseed.
> > > > 
> > > > Cheers,
> > > > Babis
> > > 
> > > Now I'm lost.
> > > when you write, e.g.:
> > > 4. vcpu 0: get getrandom() entropy and cache epoch value
> > > how does vcpu access the epoch?
> > 
> > The kernel provides a user space API to map a pointer to the epoch
> > value. User space then caches its value and checks it every time it
> > needs to make sure that no entropy leak has happened before using
> > cached kernel entropy.
> > 
> > virtio-rng driver adds a copy on leak command to the queue for
> > increasing this value (that's what we are speaking about in this thread).
> > But other systems might want to report "leaks", such as random.c
> > itself.
> > 
> > Cheers,
> > Babis
> 
> Hey Michael, does this explain the flow of things?
> 
> To summarize, end-to-end things work like this:
> 
> 1. kernel allocates a page where an epoch value is stored.
> 2. User-space can mmap this page and look for changes in its value to
>     know when it needs to re-seed its PRNGs.
> 3. virtio-rng driver gets a hold of the address of the page and programs
>     device to update it when entropy is leaked
> 4. Other kernel sub-systems can do the same if they have a concept of
>     entropy leaking. For example, random.c might want to do this
> periodically
> 
> Regarding your suggestion for MAP/UNMAP buffers, I think it could work
> if it was working with kernel addresses as well. The device doesn't care
> about user vs kernel space guest addresses after all. However, if we were to
> use such a mechanism, it seems that we don't need the two leak queues
> anyway.
> 
> So, it sounds like we can have a setup with one leak queue only. If we use a
> MAP/UNMAP type of buffer then all is fine. If we use normal buffers, we can
> add the requirement that the VMM needs to refuse to take a snapshot if there
> is not a copy on leak buffer in the queue.
> 
> Thoughts?
> 
> Cheers,
> Babis

Lost context a bit. I'll think it over. Thanks!