Re: propagating vmgenid outward and upward

[Jason A. Donenfeld]

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;