From: Kameron Carr <[email protected]> Sent: Thursday, October 1, 2026 3:11 PM > > 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 <[email protected]>
Reviewed-by: Michael Kelley <[email protected]> > --- > 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
