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Subject: Re: [virtio-dev] [PATCH RFC 3/3] rng: leak detection support
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 3I 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, BabisYes 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, BabisIt 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, BabisHeh 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
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