| Message ID | 20230823090107.65749-1-bchalios@amazon.es (mailing list archive) |
|---|---|
| Headers | show |
| Series | Propagating reseed notifications to user space | expand |
Hello all, On 23/8/23 11:01, Babis Chalios wrote: > This is an RFC, so that we can discuss whether the proposed ABI works. > Also, I'd like to hear people's opinion on the internal registration > API, 8/24 split etc. If we decide that this approach works, I 'm happy > to add documentation for it, with examples on how user space can make > use of it. Some time has passed since I sent this and I haven't received any comments, so I assume people are happy with the proposed API. I will work on adding documentation and examples on how user space can use this and send a v1. Cheers, Babis
On Mon, Sep 04, 2023 at 03:44:48PM +0200, Babis Chalios wrote: > Hello all, > > On 23/8/23 11:01, Babis Chalios wrote: > > This is an RFC, so that we can discuss whether the proposed ABI works. > > Also, I'd like to hear people's opinion on the internal registration > > API, 8/24 split etc. If we decide that this approach works, I 'm happy > > to add documentation for it, with examples on how user space can make > > use of it. > > Some time has passed since I sent this and I haven't received any > comments, so I assume people Nope. This still stands: https://lore.kernel.org/all/CAHmME9pxc-nO_xa=4+1CnvbnuefbRTJHxM7n817c_TPeoxzu_g@mail.gmail.com/ And honestly the constant pushing from you has in part been demotivating.
Hi Jason, On 4/9/23 16:42, Jason A. Donenfeld wrote: > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe. > > > > On Mon, Sep 04, 2023 at 03:44:48PM +0200, Babis Chalios wrote: >> Hello all, >> >> On 23/8/23 11:01, Babis Chalios wrote: >>> This is an RFC, so that we can discuss whether the proposed ABI works. >>> Also, I'd like to hear people's opinion on the internal registration >>> API, 8/24 split etc. If we decide that this approach works, I 'm happy >>> to add documentation for it, with examples on how user space can make >>> use of it. >> Some time has passed since I sent this and I haven't received any >> comments, so I assume people > Nope. This still stands: > https://lore.kernel.org/all/CAHmME9pxc-nO_xa=4+1CnvbnuefbRTJHxM7n817c_TPeoxzu_g@mail.gmail.com/ Could you elaborate on why the proposed RFC is not inline with your plan? We need to let user space know that it needs to reseed its PRNGs. It is not very clear to me, how does that interplay with having a getrandom vDSO. IOW, say we did have a vDSO getrandom, don't you think we should have such an API to notify when it needs to discard stale state, or do you think this is not the right API? > And honestly the constant pushing from you has in part been > demotivating. Cheers, Babis
[Resending in plain text only. Let's hope it reaches everyone this time :)] Hey Jason! On 04.09.23 16:42, Jason A. Donenfeld wrote: > On Mon, Sep 04, 2023 at 03:44:48PM +0200, Babis Chalios wrote: >> Hello all, >> >> On 23/8/23 11:01, Babis Chalios wrote: >>> This is an RFC, so that we can discuss whether the proposed ABI works. >>> Also, I'd like to hear people's opinion on the internal registration >>> API, 8/24 split etc. If we decide that this approach works, I 'm happy >>> to add documentation for it, with examples on how user space can make >>> use of it. >> Some time has passed since I sent this and I haven't received any >> comments, so I assume people > Nope. This still stands: > https://lore.kernel.org/all/CAHmME9pxc-nO_xa=4+1CnvbnuefbRTJHxM7n817c_TPeoxzu_g@mail.gmail.com/ To recap, that email said: > Just so you guys know, roughly the order of operations here are going to be: > > - vdso vgetrandom v+1 > - virtio fork driver > - exposing fork events to userspace > > I'll keep you posted on those. I don't quite understand both the relationship of vgetrandom to this nor how we could help. I understand how a VDSO vgetrandom could use primitives that are very similar (or maybe even identical) to this patch set. What I'm missing is why there is a dependency between them. I don't expect user space PRNGs to disappear over night, especially given all the heavy lifting and architecture specific code that vDSOs require. So if we want to build a solution that allows user space to generically solve VM snapshots, we should strive to have a mechanism that works in today's environment in addition to making the vgetrandom call safe when it emerges. The last revision of vgetrandom that I found was v14 from January. Is it still in active development? And if so, what is the status? The last fundamental comment I found in archives was this comment from Linus: > This should all be in libc. Not in the kernel with special magic vdso > support and special buffer allocations. The kernel should give good > enough support that libc can do a good job, but the kernel should > simply *not* take the approach of "libc will get this wrong, so let's > just do all the work for it". to which you replied > That buffering cannot be done safely currently -- VM forks, reseeding > semantics, and so forth. Again, discussed in the cover letter of the > patch if you'd like to engage with those ideas. My understanding is that this patch set solves exactly that problem in a way that is fully compatible with existing user space PRNGs and easy to consume as well as add support for in "Enterprise" systems for anyone who wishes to do so. So, where is v15 without VM changes standing? And why exactly should we couple vgetrandom with atomic user space reseed notifications? :) Thanks, Alex Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
Hi, Le 23/08/2023 à 11:01, Babis Chalios a écrit : > User space often implements PRNGs that use /dev/random as entropy > source. We can not expect that this randomness sources stay completely > unknown forever. For various reasons, the originating PRNG seed may > become known at which point the PRNG becomes insecure for further random > number generation. Events that can lead to that are for example fast > computers reversing the PRNG function using a number of inputs or > Virtual Machine clones which carry seed values into their clones. > > During LPC 2022 Jason, Alex, Michael and me brainstormed on how to > atomically expose a notification to user space that it should reseed. > Atomicity is key for the VM clone case. This patchset implements a > potential path to do so. > > This patchset introduces an epoch value as the means of communicating to > the guest the need to reseed. The epoch is a 32bit value with the > following form: > > RNG epoch > *-------------*---------------------* > | notifier id | epoch counter value | > *-------------*---------------------* > 8 bits 24 bits > > Changes in this value signal moments in time that PRNGs need to be > re-seeded. As a result, the intended use of the epoch from user space > PRNGs is to cache the epoch value every time they reseed using kernel > entropy, then control that its value hasn't changed before giving out > random numbers. If the value has changed the PRNG needs to reseed before > producing any more random bits. > > The API for getting hold of this value is offered through > /dev/(u)random. We introduce a new ioctl for these devices, which > creates an anonymous file descriptor. User processes can call the > ioctl() to get the anon fd and then mmap it to a single page. That page > contains the value of the epoch at offset 0. > > Naturally, random.c is the component that maintains the RNG epoch. > During initialization it allocates a single global page which holds the > epoch value. Moreover, it exposes an API to kernel subsystems > (notifiers) which can report events that require PRNG reseeding. > Notifiers register with random.c and receive an 8-bit notifier id (up to > 256 subscribers should be enough) and a pointer to the epoch. Notifying, > then, is equivalent to writing in the epoch address a new epoch value. > > Notifiers write epoch values that include the notifier ID on the higher > 8 bits and increasing counter values on the 24 remaining bits. This > guarantees that two notifiers cannot ever write the same epoch value, > since notificator IDs are unique. > > The first patch of this series implements the epoch mechanism. It adds > the logic in the random.c to maintain the epoch page and expose the > user space facing API. It also adds the internal API that allows kernel > systems to register as notifiers. From userspace point of view, having to open /dev/random, ioctl, and mmap() is a no-go for a (CS)PRNG embedded in libc for arc4random(). I'm biased, as I proposed to expose such seed epoch value to userspace through getrandom() directly, relying on vDSO for reasonable performances, because current's glibc arc4random() is somewhat to slow to be a general replacement rand(). See https://lore.kernel.org/all/cover.1673539719.git.ydroneaud@opteya.com/ https://lore.kernel.org/all/ae35afa5b824dc76c5ded98efcabc117e6dd3d70@opteya.com/ Reards.
Hey Yann! On 17.09.23 15:34, Yann Droneaud wrote: > > Hi, > > Le 23/08/2023 à 11:01, Babis Chalios a écrit : >> User space often implements PRNGs that use /dev/random as entropy >> source. We can not expect that this randomness sources stay completely >> unknown forever. For various reasons, the originating PRNG seed may >> become known at which point the PRNG becomes insecure for further random >> number generation. Events that can lead to that are for example fast >> computers reversing the PRNG function using a number of inputs or >> Virtual Machine clones which carry seed values into their clones. >> >> During LPC 2022 Jason, Alex, Michael and me brainstormed on how to >> atomically expose a notification to user space that it should reseed. >> Atomicity is key for the VM clone case. This patchset implements a >> potential path to do so. >> >> This patchset introduces an epoch value as the means of communicating to >> the guest the need to reseed. The epoch is a 32bit value with the >> following form: >> >> RNG epoch >> *-------------*---------------------* >> | notifier id | epoch counter value | >> *-------------*---------------------* >> 8 bits 24 bits >> >> Changes in this value signal moments in time that PRNGs need to be >> re-seeded. As a result, the intended use of the epoch from user space >> PRNGs is to cache the epoch value every time they reseed using kernel >> entropy, then control that its value hasn't changed before giving out >> random numbers. If the value has changed the PRNG needs to reseed before >> producing any more random bits. >> >> The API for getting hold of this value is offered through >> /dev/(u)random. We introduce a new ioctl for these devices, which >> creates an anonymous file descriptor. User processes can call the >> ioctl() to get the anon fd and then mmap it to a single page. That page >> contains the value of the epoch at offset 0. >> >> Naturally, random.c is the component that maintains the RNG epoch. >> During initialization it allocates a single global page which holds the >> epoch value. Moreover, it exposes an API to kernel subsystems >> (notifiers) which can report events that require PRNG reseeding. >> Notifiers register with random.c and receive an 8-bit notifier id (up to >> 256 subscribers should be enough) and a pointer to the epoch. Notifying, >> then, is equivalent to writing in the epoch address a new epoch value. >> >> Notifiers write epoch values that include the notifier ID on the higher >> 8 bits and increasing counter values on the 24 remaining bits. This >> guarantees that two notifiers cannot ever write the same epoch value, >> since notificator IDs are unique. >> >> The first patch of this series implements the epoch mechanism. It adds >> the logic in the random.c to maintain the epoch page and expose the >> user space facing API. It also adds the internal API that allows kernel >> systems to register as notifiers. > > From userspace point of view, having to open /dev/random, ioctl, and > mmap() > is a no-go for a (CS)PRNG embedded in libc for arc4random(). Could you please elaborate on why it's a no-go? With any approach we take, someone somewhere needs to map and expose data to user space that we are in a new "epoch". With this patch set, you do that explicitly from user space through an fd that you keep open plus an mmap that you keep active. With vgetrandom, the kernel does it implicitly for you. So with this patch set's approach, the first call to arc4random() would need to establish the epoch mmap and leave it open. After that epoch handling is (almost) free - it's just a 32bit value compare. Are you saying that there is a problem with keeping track of that additional state? As mentioned above, we need to keep track of some state somewhere: Either in the vdso plus kernel page map logic or in the library that consumes epochs. If this is the problem, maybe the fundamental issue is that arc4random() assumes you always have everything in place to receive randomness without a handle that could go through an open/close (init/destroy) cycle? I suppose you could change that? > I'm biased, as I proposed to expose such seed epoch value to userspace > through > getrandom() directly, relying on vDSO for reasonable performances, > because > current's glibc arc4random() is somewhat to slow to be a general > replacement > rand(). > > See > https://lore.kernel.org/all/cover.1673539719.git.ydroneaud@opteya.com/ > https://lore.kernel.org/all/ae35afa5b824dc76c5ded98efcabc117e6dd3d70@opteya.com/ > There are more problems with coupling epochs to the vgetrandom approach: Not everyone will want to or can use Linux's rng as the sole source of entropy for various reasons (NIST, FIPS, TLS recommendations to not rely on a single source, real time requirements, etc) but still require knowledge of epoch changes. That means we need an alternative path for these applications regardless. May as well start with that :). If we then still conclude that vgetrandom is the best path forward to accelerate access to /dev/urandom in user space, we can just always map this patch set's epoch page into the vDSO range and then make vgetrandom consume it, similar to how a user space library would. I genuinely don't understand how vgetrandom and this patch set contradict each other. Alex Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
User space often implements PRNGs that use /dev/random as entropy source. We can not expect that this randomness sources stay completely unknown forever. For various reasons, the originating PRNG seed may become known at which point the PRNG becomes insecure for further random number generation. Events that can lead to that are for example fast computers reversing the PRNG function using a number of inputs or Virtual Machine clones which carry seed values into their clones. During LPC 2022 Jason, Alex, Michael and me brainstormed on how to atomically expose a notification to user space that it should reseed. Atomicity is key for the VM clone case. This patchset implements a potential path to do so. This patchset introduces an epoch value as the means of communicating to the guest the need to reseed. The epoch is a 32bit value with the following form: RNG epoch *-------------*---------------------* | notifier id | epoch counter value | *-------------*---------------------* 8 bits 24 bits Changes in this value signal moments in time that PRNGs need to be re-seeded. As a result, the intended use of the epoch from user space PRNGs is to cache the epoch value every time they reseed using kernel entropy, then control that its value hasn't changed before giving out random numbers. If the value has changed the PRNG needs to reseed before producing any more random bits. The API for getting hold of this value is offered through /dev/(u)random. We introduce a new ioctl for these devices, which creates an anonymous file descriptor. User processes can call the ioctl() to get the anon fd and then mmap it to a single page. That page contains the value of the epoch at offset 0. Naturally, random.c is the component that maintains the RNG epoch. During initialization it allocates a single global page which holds the epoch value. Moreover, it exposes an API to kernel subsystems (notifiers) which can report events that require PRNG reseeding. Notifiers register with random.c and receive an 8-bit notifier id (up to 256 subscribers should be enough) and a pointer to the epoch. Notifying, then, is equivalent to writing in the epoch address a new epoch value. Notifiers write epoch values that include the notifier ID on the higher 8 bits and increasing counter values on the 24 remaining bits. This guarantees that two notifiers cannot ever write the same epoch value, since notificator IDs are unique. The first patch of this series implements the epoch mechanism. It adds the logic in the random.c to maintain the epoch page and expose the user space facing API. It also adds the internal API that allows kernel systems to register as notifiers. Based on this API, we can implement notifiers. As example notifier, this patch set only contains a virtio-rng backend. If we agree on this path forward, the natural next notifier would be an in-kernel timer with a user settable frequency that allows the kernel to notify all its user space applications to reseed regularly. Michael sent out a proposal for an extension to virtio-rng [1] which we can use to turn virtio-rng devices to RNG epoch notifiers. Briefly, the proposal defines the concept of "entropy leaks", i.e. events in time that cause entropy to decrease, such as VM snapshots. A virtio-rng driver can program the device to perform a number of operations on guest memory when entropy leaks happen. The proposal defines two possible operations: * "fill-on-leak": Fill a buffer with random bytes * "copy-on-leak": Copy a source buffer in a destination buffer The second patch implements the entropy leak reporting feature of virtio-rng and registers virtio-rng devices as epoch notifiers with random.c. It ensures that one copy-on-leak operation is always in-flight to increase the epoch value when a VM snapshot occurs. The intention is that the VMM will perform these operations before resuming vCPUs, so that user space processes will observe the changes in the epoch value atomically. Additionally, it always keeps a fill-on-leak command in the queue, so that we can get some fresh entropy when VM snapshots occur. This is an RFC, so that we can discuss whether the proposed ABI works. Also, I'd like to hear people's opinion on the internal registration API, 8/24 split etc. If we decide that this approach works, I 'm happy to add documentation for it, with examples on how user space can make use of it. [1] https://lore.kernel.org/virtio-dev/20221121162756.350032-1-mst@redhat.com/ Babis Chalios (2): random: emit reseed notifications for PRNGs virtio-rng: implement entropy leak feature drivers/char/hw_random/virtio-rng.c | 189 +++++++++++++++++++++++++++- drivers/char/random.c | 147 ++++++++++++++++++++++ include/linux/random.h | 28 +++++ include/uapi/linux/random.h | 11 ++ include/uapi/linux/virtio_rng.h | 3 + 5 files changed, 373 insertions(+), 5 deletions(-)