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(-)

-- 
2.40.1

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.

-- 
Yann Droneaud

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

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.

[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 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