Hey folks,
Having finally wrapped up development of the initial vmgenid driver, I
thought I'd pull together some thoughts on vmgenid, notification, and
propagating, from disjointed conversations I've had with a few of you
over the last several weeks.
The basic problem is: VMs can be cloned, forked, rewound, or
snapshotted, and when this happens, a) the RNG needs to reseed itself,
and b) cryptographic algorithms that are not reuse resistant need to
reinitialize in one way or another. For 5.18, we're handling (a) via the
new vmgenid driver, which implements a spec from Microsoft, whereby the
driver receives ACPI notifications when a 16 byte unique value changes.
The vmgenid driver basically works, though it is racy, because that ACPI
notification can arrive after the system is already running again. This
race is even worse on Windows, where they kick the notification into a
worker thread, which then publishes it upward elsewhere to another async
mechanism, and eventually it hits the RNG and various userspace apps.
On Linux it's not that bad -- we reseed immediately upon receiving the
notification -- but it still inherits this same "push"-model deficiency,
which a "pull"-model would not have.
If we had a "pull" model, rather than just expose a 16-byte unique
identifier, the vmgenid virtual hardware would _also_ expose a
word-sized generation counter, which would be incremented every time the
unique ID changed. Then, every time we would touch the RNG, we'd simply
do an inexpensive check of this memremap()'d integer, and reinitialize
with the unique ID if the integer changed. In this way, the race would
be entirely eliminated. We would then be able to propagate this outwards
to other drivers, by just exporting an extern symbol, in the manner of
`jiffies`, and propagate it upwards to userspace, by putting it in the
vDSO, in the manner of gettimeofday. And like that, there'd be no
terrible async thing and things would work pretty easily.
But that's not what we have, because Microsoft didn't collaborate with
anybody on this, and now it's implemented in several hypervisors. Given
that I'm already spending considerable time working on the RNG, entirely
without funding, somehow I'm not super motivated to lead a
cross-industry political effort to change Microsoft's vmgenid spec.
Maybe somebody else has an appetite for this, but either way, those
changes would be several years off at best.
So given we have a "push"-model mechanism, there are two problems to
tackle, perhaps in the same way, perhaps in a different way:
A) Outwards propagation toward other kernel drivers: in this case, I
have in mind WireGuard, naturally, which very much needs to clear its
existing sessions when VMs are forked.
B) Upwards propagation to userspace: in this case, we handle the
concerns of the Amazon engineers on this thread who broached this
topic a few years ago, in which s2n, their TLS library, wants to
reinitialize its userspace RNG (a silly thing, but I digress) and
probably clear session keys too, for the same good reason as
WireGuard.
For (A), at least wearing my WireGuard-maintainer hat, there is an easy
way and there is a "race-free" way. I use scare quotes there because
we're still in a "push"-model, which means it's still racy no matter
what.
The faux "race-free" way involves having `extern u32 rng_vm_generation;`
or similar in random.h, and then everything that generates a session key
would snapshot this value, and every time a session key is used, a
comparison would be made. This works, but given that we're going to be
racy no matter what, I think I'd prefer avoiding the extra code in the
hot path and extra per-session storage. It seems like that'd involve a
lot of fiddly engineering for no real world benefit.
The easy way, and the way that I think I prefer, would be to just have a
sync notifier_block for this, just like we have with
register_pm_notifier(). From my perspective, it'd be simplest to just
piggy back on the already existing PM notifier with an extra event,
PM_POST_VMFORK, which would join the existing set of 7, following
PM_POST_RESTORE. I think that'd be coherent. However, if the PM people
don't want to play ball, we could always come up with our own
notifier_block. But I don't see the need. Plus, WireGuard *already*
uses the PM notifier for clearing keys, so code-wise for my use case,
that'd amount adding another case for PM_POST_VMFORK, in addition to the
currently existing PM_HIBERNATION_PREPARE and PM_SUSPEND_PREPARE cases,
which all would be treated the same way. Ezpz. So if that sounds like an
interesting thing to the PM people, I think I'd like to propose a patch
for that, possibly even for 5.18, given that it'd be very straight-
forward.
For (B), it's a little bit trickier. But I think our options follow the
same rubric. We can expose a generation counter in the vDSO, with
semantics akin to the extern integer I described above. Or we could
expose that counter in a file that userspace could poll() on and receive
notifications that way. Or perhaps a third way. I'm all ears here.
Alex's team from Amazon last year proposed something similar to the vDSO
idea, except using mmap on a sysfs file, though from what I can tell,
that wound up being kind of complicated. Due to the fact that we're
_already_ racy, I think I'm most inclined at this point toward the
poll() approach for the same reasons as I prefer a notifier_block. But
on userspace I could be convinced otherwise, and I'd be interested in
totally different ideas here too.
Another thing I should note is that, while I'm not currently leaning
toward it, the vDSO approach also ties into interesting discussions
about userspace RNGs (generally a silly idea), and their need for things
like fork detection and also learning when the kernel RNG was last
reseeded. So cracking open the vDSO book might invite all sorts of other
interesting questions and discussions, which may be productive or may be
a humongous distraction. (Also, again, I'm not super enthusiastic about
userspace RNGs.)
Also, there is an interesting question to decide with regards to
userspace, which is whether the vmgenid driver should expose its unique
ID to userspace, as Alex requested on an earlier thread. I am actually
sort of opposed to this. That unique ID may or may not be secret and
entropic; if it isn't, the crypto is designed to not be impacted
negatively, but if it is, we should keep it secret. So, rather, I think
the correct flow is that userspace simply calls getrandom() upon
learning that the VM forked, which is guaranteed to have been
reinitialized already by add_vmfork_randomness(), and that will
guarantee a value that is unique to the VM, without having to actually
expose that value.
So, anyway, this is more or less where my thinking on this matter is.
Would be happy to hear some fresh ideas here too.
Regards,
Jason
On Tue, Mar 01, 2022 at 07:24:11PM +0100, Jason A. Donenfeld wrote:
> Hi Greg,
>
> On Tue, Mar 1, 2022 at 7:01 PM Greg KH <[email protected]> wrote:
> > A notifier block like this makes sense, but why tie onto the PM_ stuff?
> > This isn't power management issues, it's a system-wide change that I am
> > sure others will want to know about that doesn't reflect any power
> > changes.
> >
> > As much as I hate adding new notifiers in the kernel, that might be all
> > you need here.
>
> You might indeed be right. I guess I was thinking that "resuming from
> suspend" and "resuming from a VM fork" are kind of the same thing.
> There _is_ a certain kind of similarity between the two. I was hoping
> if the similarity was a strong enough one, maybe it'd make sense to do
> them together rather than adding another notifier. But I suppose you
> disagree, and it sounds like Rafael might too --
> <https://lore.kernel.org/lkml/CAJZ5v0g+GihH_b9YvwuHzdrUVNGXOeabOznDC1vK6qLi8gtSTQ@mail.gmail.com/>.
Hey, nice, we agree! :)
> Code-wise for me with WireGuard it's of course appealing to treat them
> the same, since it's like a one line change, but if I need to add a
> new notifier call there, it's not the end of the world.
I know there are other places in the kernel that would like to be
notified when they have been moved to another machine so that they can
do things like determine if the CPU functionality has changed (or not),
and perhaps do other types of device reconfiguration. Right now the
kernel does not have any way of knowing this, so it makes sense that if
the platform (i.e. ACPI here) has a way of creating such a event, it
should and then we can start tieing in other subsystems to use it
as-needed.
thanks,
greg k-h
On Tue, Mar 1, 2022 at 8:41 PM Greg KH <[email protected]> wrote:
>
> On Tue, Mar 01, 2022 at 07:24:11PM +0100, Jason A. Donenfeld wrote:
> > Hi Greg,
> >
> > On Tue, Mar 1, 2022 at 7:01 PM Greg KH <[email protected]> wrote:
> > > A notifier block like this makes sense, but why tie onto the PM_ stuff?
> > > This isn't power management issues, it's a system-wide change that I am
> > > sure others will want to know about that doesn't reflect any power
> > > changes.
> > >
> > > As much as I hate adding new notifiers in the kernel, that might be all
> > > you need here.
> >
> > You might indeed be right. I guess I was thinking that "resuming from
> > suspend" and "resuming from a VM fork" are kind of the same thing.
> > There _is_ a certain kind of similarity between the two. I was hoping
> > if the similarity was a strong enough one, maybe it'd make sense to do
> > them together rather than adding another notifier. But I suppose you
> > disagree, and it sounds like Rafael might too --
> > <https://lore.kernel.org/lkml/CAJZ5v0g+GihH_b9YvwuHzdrUVNGXOeabOznDC1vK6qLi8gtSTQ@mail.gmail.com/>.
>
> Hey, nice, we agree! :)
It is now done and posted here:
https://lore.kernel.org/lkml/[email protected]/
Jason
On Wed, Mar 02, 2022 at 04:36:49PM +0100, Jason A. Donenfeld wrote:
> Hi Michael,
>
> On Wed, Mar 02, 2022 at 10:20:25AM -0500, Michael S. Tsirkin wrote:
> > So writing some code:
> >
> > 1:
> > put plaintext in a buffer
> > put a key in a buffer
> > put the nonce for that encryption in a buffer
> >
> > if vm gen id != stored vm gen id
> > stored vm gen id = vm gen id
> > goto 1
> >
> > I think this is race free, but I don't see why does it matter whether we
> > read gen id atomically or not.
>
> Because that 16 byte read of vmgenid is not atomic. Let's say you read
> the first 8 bytes, and then the VM is forked. In the forked VM, the next
> 8 bytes are the same as last time, but the first 8 bytes, which you
> already read, have changed. In that case, your != becomes a ==, and the
> test fails.
>
> This is one of those fundamental things of "unique ID" vs "generation
> counter word".
>
> Anyway, per your request in your last email, I wrote some code for this,
> which may or may not be totally broken, and only works on 64-bit x86,
> which is really the best possible case in terms of performance. And even
> so, it's not great.
>
> Jason
>
> --------8<------------------------
>
> diff --git a/drivers/net/wireguard/noise.c b/drivers/net/wireguard/noise.c
> index 720952b92e78..250b8973007d 100644
> --- a/drivers/net/wireguard/noise.c
> +++ b/drivers/net/wireguard/noise.c
> @@ -106,6 +106,7 @@ static struct noise_keypair *keypair_create(struct wg_peer *peer)
> keypair->entry.type = INDEX_HASHTABLE_KEYPAIR;
> keypair->entry.peer = peer;
> kref_init(&keypair->refcount);
> + keypair->vmgenid = vmgenid_read_atomic();
> return keypair;
> }
>
> diff --git a/drivers/net/wireguard/noise.h b/drivers/net/wireguard/noise.h
> index c527253dba80..0add240a14a0 100644
> --- a/drivers/net/wireguard/noise.h
> +++ b/drivers/net/wireguard/noise.h
> @@ -27,10 +27,13 @@ struct noise_symmetric_key {
> bool is_valid;
> };
>
> +extern __uint128_t vmgenid_read_atomic(void);
> +
> struct noise_keypair {
> struct index_hashtable_entry entry;
> struct noise_symmetric_key sending;
> atomic64_t sending_counter;
> + __uint128_t vmgenid;
> struct noise_symmetric_key receiving;
> struct noise_replay_counter receiving_counter;
> __le32 remote_index;
> diff --git a/drivers/net/wireguard/send.c b/drivers/net/wireguard/send.c
> index 5368f7c35b4b..40d016be59e3 100644
> --- a/drivers/net/wireguard/send.c
> +++ b/drivers/net/wireguard/send.c
> @@ -381,6 +381,9 @@ void wg_packet_send_staged_packets(struct wg_peer *peer)
> goto out_invalid;
> }
>
> + if (keypair->vmgenid != vmgenid_read_atomic())
> + goto out_invalid;
> +
> packets.prev->next = NULL;
> wg_peer_get(keypair->entry.peer);
> PACKET_CB(packets.next)->keypair = keypair;
I don't think we care about an atomic read here. All data is in buffer
by this point, if it did not fork before that then we are ok, even
if it forks during the read.
We probably do need a memory barrier to make sure all writes complete
before the read of vmgenid, I'm not sure which kind - I think hypervisor
can be trusted to do a full CPU barrier on fork so probably just a
compiler barrier.
> diff --git a/drivers/virt/vmgenid.c b/drivers/virt/vmgenid.c
> index 0ae1a39f2e28..c122fae1d494 100644
> --- a/drivers/virt/vmgenid.c
> +++ b/drivers/virt/vmgenid.c
> @@ -21,6 +21,21 @@ struct vmgenid_state {
> u8 this_id[VMGENID_SIZE];
> };
>
> +static __uint128_t *val;
> +
> +__uint128_t vmgenid_read_atomic(void)
> +{
> + __uint128_t ret = 0;
> + if (!val)
> + return 0;
> + asm volatile("lock cmpxchg16b %1"
> + : "+A"(ret)
> + : "m"(*val), "b"(0), "c"(0)
> + : "cc");
> + return ret;
> +}
> +EXPORT_SYMBOL(vmgenid_read_atomic);
> +
> static int vmgenid_add(struct acpi_device *device)
> {
> struct acpi_buffer parsed = { ACPI_ALLOCATE_BUFFER };
> @@ -50,6 +65,7 @@ static int vmgenid_add(struct acpi_device *device)
> phys_addr = (obj->package.elements[0].integer.value << 0) |
> (obj->package.elements[1].integer.value << 32);
> state->next_id = devm_memremap(&device->dev, phys_addr, VMGENID_SIZE, MEMREMAP_WB);
> + val = (__uint128_t *)state->next_id;
> if (IS_ERR(state->next_id)) {
> ret = PTR_ERR(state->next_id);
> goto out;
Hi Michael,
On Wed, Mar 02, 2022 at 10:20:25AM -0500, Michael S. Tsirkin wrote:
> So writing some code:
>
> 1:
> put plaintext in a buffer
> put a key in a buffer
> put the nonce for that encryption in a buffer
>
> if vm gen id != stored vm gen id
> stored vm gen id = vm gen id
> goto 1
>
> I think this is race free, but I don't see why does it matter whether we
> read gen id atomically or not.
Because that 16 byte read of vmgenid is not atomic. Let's say you read
the first 8 bytes, and then the VM is forked. In the forked VM, the next
8 bytes are the same as last time, but the first 8 bytes, which you
already read, have changed. In that case, your != becomes a ==, and the
test fails.
This is one of those fundamental things of "unique ID" vs "generation
counter word".
Anyway, per your request in your last email, I wrote some code for this,
which may or may not be totally broken, and only works on 64-bit x86,
which is really the best possible case in terms of performance. And even
so, it's not great.
Jason
--------8<------------------------
diff --git a/drivers/net/wireguard/noise.c b/drivers/net/wireguard/noise.c
index 720952b92e78..250b8973007d 100644
--- a/drivers/net/wireguard/noise.c
+++ b/drivers/net/wireguard/noise.c
@@ -106,6 +106,7 @@ static struct noise_keypair *keypair_create(struct wg_peer *peer)
keypair->entry.type = INDEX_HASHTABLE_KEYPAIR;
keypair->entry.peer = peer;
kref_init(&keypair->refcount);
+ keypair->vmgenid = vmgenid_read_atomic();
return keypair;
}
diff --git a/drivers/net/wireguard/noise.h b/drivers/net/wireguard/noise.h
index c527253dba80..0add240a14a0 100644
--- a/drivers/net/wireguard/noise.h
+++ b/drivers/net/wireguard/noise.h
@@ -27,10 +27,13 @@ struct noise_symmetric_key {
bool is_valid;
};
+extern __uint128_t vmgenid_read_atomic(void);
+
struct noise_keypair {
struct index_hashtable_entry entry;
struct noise_symmetric_key sending;
atomic64_t sending_counter;
+ __uint128_t vmgenid;
struct noise_symmetric_key receiving;
struct noise_replay_counter receiving_counter;
__le32 remote_index;
diff --git a/drivers/net/wireguard/send.c b/drivers/net/wireguard/send.c
index 5368f7c35b4b..40d016be59e3 100644
--- a/drivers/net/wireguard/send.c
+++ b/drivers/net/wireguard/send.c
@@ -381,6 +381,9 @@ void wg_packet_send_staged_packets(struct wg_peer *peer)
goto out_invalid;
}
+ if (keypair->vmgenid != vmgenid_read_atomic())
+ goto out_invalid;
+
packets.prev->next = NULL;
wg_peer_get(keypair->entry.peer);
PACKET_CB(packets.next)->keypair = keypair;
diff --git a/drivers/virt/vmgenid.c b/drivers/virt/vmgenid.c
index 0ae1a39f2e28..c122fae1d494 100644
--- a/drivers/virt/vmgenid.c
+++ b/drivers/virt/vmgenid.c
@@ -21,6 +21,21 @@ struct vmgenid_state {
u8 this_id[VMGENID_SIZE];
};
+static __uint128_t *val;
+
+__uint128_t vmgenid_read_atomic(void)
+{
+ __uint128_t ret = 0;
+ if (!val)
+ return 0;
+ asm volatile("lock cmpxchg16b %1"
+ : "+A"(ret)
+ : "m"(*val), "b"(0), "c"(0)
+ : "cc");
+ return ret;
+}
+EXPORT_SYMBOL(vmgenid_read_atomic);
+
static int vmgenid_add(struct acpi_device *device)
{
struct acpi_buffer parsed = { ACPI_ALLOCATE_BUFFER };
@@ -50,6 +65,7 @@ static int vmgenid_add(struct acpi_device *device)
phys_addr = (obj->package.elements[0].integer.value << 0) |
(obj->package.elements[1].integer.value << 32);
state->next_id = devm_memremap(&device->dev, phys_addr, VMGENID_SIZE, MEMREMAP_WB);
+ val = (__uint128_t *)state->next_id;
if (IS_ERR(state->next_id)) {
ret = PTR_ERR(state->next_id);
goto out;
Hey again,
On Tue, Mar 01, 2022 at 04:42:47PM +0100, Jason A. Donenfeld wrote:
> For (B), it's a little bit trickier. But I think our options follow the
> same rubric. We can expose a generation counter in the vDSO, with
> semantics akin to the extern integer I described above. Or we could
> expose that counter in a file that userspace could poll() on and receive
> notifications that way. Or perhaps a third way. I'm all ears here.
> Alex's team from Amazon last year proposed something similar to the vDSO
> idea, except using mmap on a sysfs file, though from what I can tell,
> that wound up being kind of complicated. Due to the fact that we're
> _already_ racy, I think I'm most inclined at this point toward the
> poll() approach for the same reasons as I prefer a notifier_block. But
> on userspace I could be convinced otherwise, and I'd be interested in
> totally different ideas here too.
I implemented the poll() case here in 15 lines of code and found it
remarkably simple to do:
https://lore.kernel.org/lkml/[email protected]/
This is just a PoC/RFC for the sake of having something tangible to look
at for this thread. It is notable to me, though, that implementing this
was so minimal.
Regards,
Jason
On Wed, Mar 02, 2022 at 02:55:29PM +0100, Jason A. Donenfeld wrote:
> Hi Michael,
>
> On Wed, Mar 02, 2022 at 07:58:33AM -0500, Michael S. Tsirkin wrote:
> > > There's also the atomicity aspect, which I think makes your benchmark
> > > not quite accurate. Those 16 bytes could change between the first and
> > > second word (or between the Nth and N+1th word for N<=3 on 32-bit).
> > > What if in that case the word you read second doesn't change, but the
> > > word you read first did? So then you find yourself having to do a
> > > hi-lo-hi dance.
> > > And then consider the 32-bit case, where that's even
> > > more annoying. This is just one of those things that comes up when you
> > > compare the semantics of a "large unique ID" and "word-sized counter",
> > > as general topics. (My suggestion is that vmgenid provide both.)
> >
> > I don't see how this matters for any applications at all. Feel free to
> > present a case that would be race free with a word but not a 16
> > byte value, I could not imagine one. It's human to err of course.
>
> Word-size reads happen all at once on systems that Linux supports,
> whereas this is not the case for 16 bytes (with a few niche exceptions
> like cmpxchg16b and such). If you read the counter atomically, you can
> check to see whether it's changed just after encrypting but before
> transmitting and not transmit if it has changed, and voila, no race.
> With 16 bytes, synchronization of that read is pretty tricky (though
> maybe not all together impossible), because, as I mentioned, the first
> word might have changed by the time you read a matching second word. I'm
> sure you're familiar with the use of seqlocks in the kernel for solving
> a somewhat related problem.
>
> Jason
I just don't see how "value changed while it was read" is so different
from "value changed one clock after it was read". Since we don't detect
the latter I don't see why we should worry about the former. What I
don't have here is how would a code reading the value look. It might
help to write some pseudo code to show that, but I'd say it makes more
sense to just code the read up even just so the overhead of the current
interface can be roughtly measured.
--
MST
Hi Michael,
On Wed, Mar 02, 2022 at 07:58:33AM -0500, Michael S. Tsirkin wrote:
> > There's also the atomicity aspect, which I think makes your benchmark
> > not quite accurate. Those 16 bytes could change between the first and
> > second word (or between the Nth and N+1th word for N<=3 on 32-bit).
> > What if in that case the word you read second doesn't change, but the
> > word you read first did? So then you find yourself having to do a
> > hi-lo-hi dance.
> > And then consider the 32-bit case, where that's even
> > more annoying. This is just one of those things that comes up when you
> > compare the semantics of a "large unique ID" and "word-sized counter",
> > as general topics. (My suggestion is that vmgenid provide both.)
>
> I don't see how this matters for any applications at all. Feel free to
> present a case that would be race free with a word but not a 16
> byte value, I could not imagine one. It's human to err of course.
Word-size reads happen all at once on systems that Linux supports,
whereas this is not the case for 16 bytes (with a few niche exceptions
like cmpxchg16b and such). If you read the counter atomically, you can
check to see whether it's changed just after encrypting but before
transmitting and not transmit if it has changed, and voila, no race.
With 16 bytes, synchronization of that read is pretty tricky (though
maybe not all together impossible), because, as I mentioned, the first
word might have changed by the time you read a matching second word. I'm
sure you're familiar with the use of seqlocks in the kernel for solving
a somewhat related problem.
Jason
On 01.03.22 16:42, Jason A. Donenfeld wrote:
>
> Hey folks,
>
> Having finally wrapped up development of the initial vmgenid driver, I
> thought I'd pull together some thoughts on vmgenid, notification, and
> propagating, from disjointed conversations I've had with a few of you
> over the last several weeks.
>
> The basic problem is: VMs can be cloned, forked, rewound, or
> snapshotted, and when this happens, a) the RNG needs to reseed itself,
> and b) cryptographic algorithms that are not reuse resistant need to
> reinitialize in one way or another. For 5.18, we're handling (a) via the
> new vmgenid driver, which implements a spec from Microsoft, whereby the
> driver receives ACPI notifications when a 16 byte unique value changes.
>
> The vmgenid driver basically works, though it is racy, because that ACPI
> notification can arrive after the system is already running again. This
I believe enough people already pointed out that this assumption is
incorrect. The thing that is racy about VMGenID is the interrupt based
notification. The actual identifier is updated before the VM resumes
from its clone operation, so if you match on that you will know whether
you are in a new or old world. And that is enough to create
transactions: Save the identifier before a "crypto transaction",
validate before you finish, if they don't match, abort, reseed and replay.
> race is even worse on Windows, where they kick the notification into a
> worker thread, which then publishes it upward elsewhere to another async
> mechanism, and eventually it hits the RNG and various userspace apps.
> On Linux it's not that bad -- we reseed immediately upon receiving the
> notification -- but it still inherits this same "push"-model deficiency,
> which a "pull"-model would not have.
>
> If we had a "pull" model, rather than just expose a 16-byte unique
> identifier, the vmgenid virtual hardware would _also_ expose a
> word-sized generation counter, which would be incremented every time the
> unique ID changed. Then, every time we would touch the RNG, we'd simply
> do an inexpensive check of this memremap()'d integer, and reinitialize
> with the unique ID if the integer changed. In this way, the race would
> be entirely eliminated. We would then be able to propagate this outwards
> to other drivers, by just exporting an extern symbol, in the manner of
> `jiffies`, and propagate it upwards to userspace, by putting it in the
> vDSO, in the manner of gettimeofday. And like that, there'd be no
> terrible async thing and things would work pretty easily.
>
> But that's not what we have, because Microsoft didn't collaborate with
> anybody on this, and now it's implemented in several hypervisors. Given
> that I'm already spending considerable time working on the RNG, entirely
> without funding, somehow I'm not super motivated to lead a
> cross-industry political effort to change Microsoft's vmgenid spec.
> Maybe somebody else has an appetite for this, but either way, those
> changes would be several years off at best.
>
> So given we have a "push"-model mechanism, there are two problems to
> tackle, perhaps in the same way, perhaps in a different way:
>
> A) Outwards propagation toward other kernel drivers: in this case, I
> have in mind WireGuard, naturally, which very much needs to clear its
> existing sessions when VMs are forked.
>
> B) Upwards propagation to userspace: in this case, we handle the
> concerns of the Amazon engineers on this thread who broached this
> topic a few years ago, in which s2n, their TLS library, wants to
> reinitialize its userspace RNG (a silly thing, but I digress) and
> probably clear session keys too, for the same good reason as
> WireGuard.
>
> For (A), at least wearing my WireGuard-maintainer hat, there is an easy
> way and there is a "race-free" way. I use scare quotes there because
> we're still in a "push"-model, which means it's still racy no matter
> what.
>
> The faux "race-free" way involves having `extern u32 rng_vm_generation;`
> or similar in random.h, and then everything that generates a session key
> would snapshot this value, and every time a session key is used, a
> comparison would be made. This works, but given that we're going to be
> racy no matter what, I think I'd prefer avoiding the extra code in the
> hot path and extra per-session storage. It seems like that'd involve a
> lot of fiddly engineering for no real world benefit.
>
> The easy way, and the way that I think I prefer, would be to just have a
> sync notifier_block for this, just like we have with
> register_pm_notifier(). From my perspective, it'd be simplest to just
> piggy back on the already existing PM notifier with an extra event,
> PM_POST_VMFORK, which would join the existing set of 7, following
> PM_POST_RESTORE. I think that'd be coherent. However, if the PM people
> don't want to play ball, we could always come up with our own
> notifier_block. But I don't see the need. Plus, WireGuard *already*
> uses the PM notifier for clearing keys, so code-wise for my use case,
> that'd amount adding another case for PM_POST_VMFORK, in addition to the
> currently existing PM_HIBERNATION_PREPARE and PM_SUSPEND_PREPARE cases,
> which all would be treated the same way. Ezpz. So if that sounds like an
> interesting thing to the PM people, I think I'd like to propose a patch
> for that, possibly even for 5.18, given that it'd be very straight-
> forward.
>
> For (B), it's a little bit trickier. But I think our options follow the
> same rubric. We can expose a generation counter in the vDSO, with
> semantics akin to the extern integer I described above. Or we could
> expose that counter in a file that userspace could poll() on and receive
> notifications that way. Or perhaps a third way. I'm all ears here.
> Alex's team from Amazon last year proposed something similar to the vDSO
> idea, except using mmap on a sysfs file, though from what I can tell,
> that wound up being kind of complicated. Due to the fact that we're
> _already_ racy, I think I'm most inclined at this point toward the
> poll() approach for the same reasons as I prefer a notifier_block. But
> on userspace I could be convinced otherwise, and I'd be interested in
> totally different ideas here too.
>
> Another thing I should note is that, while I'm not currently leaning
> toward it, the vDSO approach also ties into interesting discussions
> about userspace RNGs (generally a silly idea), and their need for things
> like fork detection and also learning when the kernel RNG was last
> reseeded. So cracking open the vDSO book might invite all sorts of other
> interesting questions and discussions, which may be productive or may be
> a humongous distraction. (Also, again, I'm not super enthusiastic about
> userspace RNGs.)
>
> Also, there is an interesting question to decide with regards to
> userspace, which is whether the vmgenid driver should expose its unique
> ID to userspace, as Alex requested on an earlier thread. I am actually
> sort of opposed to this. That unique ID may or may not be secret and
> entropic; if it isn't, the crypto is designed to not be impacted
> negatively, but if it is, we should keep it secret. So, rather, I think
> the correct flow is that userspace simply calls getrandom() upon
> learning that the VM forked, which is guaranteed to have been
> reinitialized already by add_vmfork_randomness(), and that will
> guarantee a value that is unique to the VM, without having to actually
> expose that value.
If you follow the logic at the beginning of the mail, you can create
something race free if you consume the hardware VMGenID counter. You can
not make it race free if you rely on the interrupt mechanism.
So following that train of thought, if you expose the hardware VMGenID
to user space, you could allow user space to act race free based on
VMGenID. That means consumers of user space RNGs could validate whether
the ID is identical between the beginning of the crypto operation and
the end.
That said, there are 2 pieces to the puzzle of user space notification:
Polling and event based. The part above solves the polling use cases -
user space libraries that just want to know whether they are now in a
new world.
However, there are more complicated cases as well. What do you do with
Samba for example? It needs to generate a new SID after the clone.
That's a super heavy operation. Do you want to have smbd constantly poll
on the VMGenID just to see whether it needs to kick off some
administrative actions?
For the event based approach, we're in the same boat as "S3 resume" - we
need a global notification mechanism that the state of the system
changed and act accordingly. That's where the systemd proposal[1] comes
in: Create inhibitors and scriptlets that get spawned when we want to
suspend and then resume-cloned later. I'm personally even ok if we just
limit that whole use case to cloning while you're in S3 only.
In that case, all we would need from the kernel is an easily readable
GenID that changes before systemd runs again after suspend: Systemd
wakes up after resume, checks if the GenID changed and if so, invokes
the unquiescing target in after the resume one.
For this particular use case we're not in the fast path, so we could
make GenID reading a syscall which checks against VMGenID. But that
won't cut it for the polling use case.
I'm also not a super big fan of putting all that logic into systemd. It
means applications need to create their own notification mechanisms to
pass that cloning notification into actual processes. Don't we have any
mechanism that applications and libraries could use to natively get an
event when the GenID changes?
Alex
[1] https://github.com/systemd/systemd/issues/20222
>
> So, anyway, this is more or less where my thinking on this matter is.
> Would be happy to hear some fresh ideas here too.
>
> Regards,
> Jason
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
On 09.03.22 23:02, Jason A. Donenfeld wrote:
> Hi Alex,
>
> On Wed, Mar 9, 2022 at 3:10 AM Alexander Graf <[email protected]> wrote:
>>> The vmgenid driver basically works, though it is racy, because that ACPI
>>> notification can arrive after the system is already running again. This
>>
>> I believe enough people already pointed out that this assumption is
>> incorrect. The thing that is racy about VMGenID is the interrupt based
>> notification.
> I'm having a hard time figuring out what's different between your
> statement and mine. I said that the race is due to the notification.
> You said that the race is due to the notification. What subtle thing
> am I missing here that would lead you to say that my assumption is
> incorrect? Or did you just misread?
The subtle difference is that you don't need to rely on the notification
to learn about the world switch. If you instead read VMGenID explicitly
without waiting for the notification, you're guaranteed to always know
whether you were cloned. That means the actual VMGenID interface is not
always racy. Just the notification part is.
So you *can* build a race-free VMGenID mechanism. You just can't build a
race-free *event based* VMGenID mechanism if you want to allow cloning
at arbitrary points in time.
>
>> The actual identifier is updated before the VM resumes
>> from its clone operation, so if you match on that you will know whether
>> you are in a new or old world. And that is enough to create
>> transactions: Save the identifier before a "crypto transaction",
>> validate before you finish, if they don't match, abort, reseed and replay.
> Right. But more than just transactions, it's useful to preventing key
> reuse vulnerabilities, in which case, you store the current identifier
> just before an ephemeral key is generated, and then subsequently check
> to see that the identifier hasn't changed before transmitting anything
> related to that key.
>
>> If you follow the logic at the beginning of the mail, you can create
>> something race free if you consume the hardware VMGenID counter. You can
>> not make it race free if you rely on the interrupt mechanism.
> Yes, as mentioned and discussed in depth before. However, your use of
> the word "counter" is problematic. Vmgenid is not a counter. It's a
> unique identifier. That means you can't compare it with a single word
> comparison but have to compare all of the 16 bytes. That seems
> potentially expensive. It's for that reason that I suggested
> augmenting the vmgenid spec with an additional word-sized _counter_
> that could be mapped into the kernels and into userspaces.
I think Michael's benchmarks show quite well that it's not all that
expensive. We're talking 2x 64bit compares within the same cache line.
You already realized that we don't need them to be atomic - just
properly barriered.
So what if we created a vsyscall that takes a buffer of "up to 16
bytes". If we later realize that an additional page with a 4 byte
counter is a viable performance optimization, we can work with MS to add
that to the spec. But the user space interface would stay identical.
>
>> So following that train of thought, if you expose the hardware VMGenID
>> to user space, you could allow user space to act race free based on
>> VMGenID. That means consumers of user space RNGs could validate whether
>> the ID is identical between the beginning of the crypto operation and
>> the end.
> Right.
>
>> However, there are more complicated cases as well. What do you do with
>> Samba for example? It needs to generate a new SID after the clone.
>> That's a super heavy operation. Do you want to have smbd constantly poll
>> on the VMGenID just to see whether it needs to kick off some
>> administrative actions?
> Were it a single word-sized integer, mapped into memory, that wouldn't
> be much of a problem at all. It could constantly read this before and
> after every operation. The problem is that it's 16 bytes and
> understandably applications don't want to deal with that clunkiness.
I don't think applications should be in the business of mapping
arbitrary locations of the vdso space to match on them. We need to build
an interface that is fast and flexible enough so they can just say "here
is a buffer, tell me if the ID changed". That's practically all you need
- on init you run that once. Later on, you invoke it every time between
finishing a crypto operation and putting it on the wire.
>
>> In that case, all we would need from the kernel is an easily readable
>> GenID that changes
> Actually, no, you need even less than that. All that's required is a
> sysfs/procfs file that can be poll()'d on. It doesn't need to have any
> content. When poll() returns readable, the VM has been forked. Then
> userspace rngs and other things like that can call getrandom() to
> receive a fresh value to mix into whatever their operation is. Since
> all we're talking about here is _event notification_, all we need is
> that event, which is what poll() provides.
>
>> I'm also not a super big fan of putting all that logic into systemd. It
>> means applications need to create their own notification mechanisms to
>> pass that cloning notification into actual processes. Don't we have any
>> mechanism that applications and libraries could use to natively get an
>> event when the GenID changes?
> Yes. poll() can do this. For the purposes of discussion, I've posted
> an implementation of this idea here:
> https://lore.kernel.org/lkml/[email protected]/
>
> What I'm sort of leaning toward is doing something like that patch,
> and then later if vmgenid ever grows an additional word-sized counter,
> moving to explore the race-free approach. Given the amount of
> programming required to actually implement the race-free approach
> (transactions and careful study of each case), the poll() file
> approach might be a medium-grade compromise for the time being.
> Evidently that's what Microsoft decided too.
I agree on the slightly racy compromise and that it's a step into the
right direction. Doing this is a no brainer IMHO and I like the proc
based poll approach.
I have an additional problem you might have an idea for with the poll
based path. In addition to the clone notification, I'd need to know at
which point everyone who was listening to a clone notification is
finished acting up it. If I spawn a tiny VM to do "work", I want to know
when it's safe to hand requests into it. How do I find out when that
point in time is?
As far as the race-free approach goes, I wouldn't get hung up on 4 byte
vs 16 byte UUID to match against. Outside of FUD that this might
potentially have performance impact (4 byte reads will have impact
too!), there's nothing that would keep us from implementing that
interface in addition to the poll today.
I'm happy to see all of these things evolve incrementally though. We can
start with the poll interface and then later implement a vsyscall that
allows transactions in hot paths.
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
On Tue, Apr 19, 2022 at 05:12:36PM +0200, Jason A. Donenfeld wrote:
> Hey Alex,
>
> On Thu, Mar 10, 2022 at 12:18 PM Alexander Graf <[email protected]> wrote:
> > I agree on the slightly racy compromise and that it's a step into the
> > right direction. Doing this is a no brainer IMHO and I like the proc
> > based poll approach.
>
> Alright. I'm going to email a more serious patch for that in the next
> few hours and you can have a look. Let's do that for 5.19.
>
> > I have an additional problem you might have an idea for with the poll
> > based path. In addition to the clone notification, I'd need to know at
> > which point everyone who was listening to a clone notification is
> > finished acting up it. If I spawn a tiny VM to do "work", I want to know
> > when it's safe to hand requests into it. How do I find out when that
> > point in time is?
>
> Seems tricky to solve. Even a count of current waiters and a
> generation number won't be sufficient, since it wouldn't take into
> account users who haven't _yet_ gotten to waiting. But maybe it's not
> the right problem to solve? Or somehow not necessary? For example, if
> the problem is a bit more constrained a solution becomes easier: you
> have a fixed/known set of readers that you know about, and you
> guarantee that they're all waiting before the fork. Then after the
> fork, they all do something to alert you in their poll()er, and you
> count up how many alerts you get until it matches the number of
> expected waiters. Would that work? It seems like anything more general
> than that is just butting heads with the racy compromise we're already
> making.
>
> Jason
I have some ideas here ... but can you explain the use-case a bit more?
--
MST