[PATCH 4/4] Drivers: hv: vmbus: Keep the ring byte counts sane for a bad index

[Kameron Carr <kameroncarr@linux.microsoft.com>]

The remaining users of the shared indices can't cause a bad memory
access with the channels a CoCo VM accepts today, but an out-of-range
index gives them a nonsense byte count. Give them defined behavior
instead.

hv_pkt_iter_first() bounds its memcpy() by hv_pkt_iter_avail(), which
is derived from write_index, and by pkt_buffer_size, which comes from
max_pkt_size and can exceed ring_datasize. A bad write index can then
make it read past the end of the ring. Only KVP has such a max_pkt_size
(16K on a 12K ring with 4K pages), and vmbus_is_valid_offer() rejects
it in isolated VMs, so this is latent. hv_pkt_iter_avail() now reports
an empty ring for a bad write index and logs it, rate-limited like
hv_ringbuffer_write(). It takes the channel instead of the ring so the
message can include the relid.

hv_get_bytes_to_read() and hv_get_bytes_to_write() now return 0 for a
bad index, so callers see nothing to read and no room to write. This
also stops hv_end_read() from reporting data that hv_pkt_iter_first()
won't return, which would keep a channel rescheduling its callback.

hv_pkt_iter_close() only uses the indices to decide whether to signal
the host, so skip the signal if either is out of range.

Signed-off-by: Kameron Carr <kameroncarr@linux.microsoft.com>
---
 drivers/hv/ring_buffer.c | 15 +++++++++++++--
 include/linux/hyperv.h   |  6 ++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 29edab9..b54a7d3 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -408,8 +408,9 @@ int hv_ringbuffer_read(struct vmbus_channel *channel,
  * This is similar to hv_get_bytes_to_read but with private
  * read index instead.
  */
-static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi)
+static u32 hv_pkt_iter_avail(const struct vmbus_channel *channel)
 {
+	const struct hv_ring_buffer_info *rbi = &channel->inbound;
 	u32 priv_read_loc = rbi->priv_read_index;
 	u32 write_loc;
 
@@ -421,6 +422,12 @@ static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi)
 	 * stale data.
 	 */
 	write_loc = virt_load_acquire(&rbi->ring_buffer->write_index);
+	if (unlikely(!hv_ringbuffer_index_valid(rbi, write_loc))) {
+		pr_err_ratelimited("inbound write index out of range: relid %u write %u size %u\n",
+				   channel->offermsg.child_relid, write_loc,
+				   rbi->ring_datasize);
+		return 0;
+	}
 
 	if (write_loc >= priv_read_loc)
 		return write_loc - priv_read_loc;
@@ -441,7 +448,7 @@ struct vmpacket_descriptor *hv_pkt_iter_first(struct vmbus_channel *channel)
 
 	hv_debug_delay_test(channel, MESSAGE_DELAY);
 
-	bytes_avail = hv_pkt_iter_avail(rbi);
+	bytes_avail = hv_pkt_iter_avail(channel);
 	if (bytes_avail < sizeof(struct vmpacket_descriptor))
 		return NULL;
 	bytes_avail = min(rbi->pkt_buffer_size, bytes_avail);
@@ -590,6 +597,10 @@ void hv_pkt_iter_close(struct vmbus_channel *channel)
 	 */
 	virt_rmb();
 	write_index = READ_ONCE(rbi->ring_buffer->write_index);
+	if (unlikely(!hv_ringbuffer_index_valid(rbi, write_index) ||
+		     !hv_ringbuffer_index_valid(rbi, start_read_index)))
+		return;
+
 	curr_write_sz = hv_ringbuffer_avail_write(rbi, rbi->priv_read_index,
 						  write_index);
 	bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index);
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 9d7d09c..fd61382 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -248,6 +248,9 @@ static inline u32 hv_get_bytes_to_read(const struct hv_ring_buffer_info *rbi)
 
 	read_loc = READ_ONCE(rbi->ring_buffer->read_index);
 	write_loc = READ_ONCE(rbi->ring_buffer->write_index);
+	if (unlikely(!hv_ringbuffer_index_valid(rbi, read_loc) ||
+		     !hv_ringbuffer_index_valid(rbi, write_loc)))
+		return 0;
 
 	return hv_ringbuffer_avail_read(rbi, read_loc, write_loc);
 }
@@ -258,6 +261,9 @@ static inline u32 hv_get_bytes_to_write(const struct hv_ring_buffer_info *rbi)
 
 	read_loc = READ_ONCE(rbi->ring_buffer->read_index);
 	write_loc = READ_ONCE(rbi->ring_buffer->write_index);
+	if (unlikely(!hv_ringbuffer_index_valid(rbi, read_loc) ||
+		     !hv_ringbuffer_index_valid(rbi, write_loc)))
+		return 0;
 
 	return hv_ringbuffer_avail_write(rbi, read_loc, write_loc);
 }

-- 
2.45.4