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;
