From a8e60890cb86a181d8216ac7e60873ba9d7d2ff9 Mon Sep 17 00:00:00 2001 From: Kameron Carr Date: Mon, 19 Oct 2026 16:09:48 -0700 Subject: [PATCH] Drivers: hv: vmbus: Compute ring byte counts from a caller-held snapshot hv_get_ringbuffer_availbytes() reads the indices directly, so a caller that also wants the index values has to read them a second time. The host can change them in between, resulting in the byte counts and the indices reflecting two different states of the ring. Replace it with hv_ringbuffer_avail_write() and hv_ringbuffer_avail_read(), which take the indices as arguments and return a single count. They are separate because most callers want only one of the two values. Use them in hv_ringbuffer_write() in place of the open-coded equivalent. hv_get_bytes_to_read(), hv_get_bytes_to_write() and hv_pkt_iter_avail() duplicated the same arithmetic, so put the helpers in include/linux/hyperv.h and use them there too. Add a hv_ringbuffer_indices_invalid() helper that bounds checks a pair of read and write indices, and use it for the check in hv_ringbuffer_write(). Convert the remaining callers to use a single snapshot: - hv_ringbuffer_get_debuginfo() now reports the indices and the computed byte counts from the same snapshot. - hv_pkt_iter_close() computes the free space from priv_read_index instead of re-reading the shared read index. The two agree unless a misbehaving host has rewritten the value. No functional change intended for a well-behaved host. Signed-off-by: Kameron Carr Reviewed-by: Michael Kelley Signed-off-by: Wei Liu --- drivers/hv/ring_buffer.c | 68 +++++++++++++--------------------------- include/linux/hyperv.h | 50 ++++++++++++++++++++++------- 2 files changed, 61 insertions(+), 57 deletions(-) diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c index 7f466c56ba0f..cda957d5d5c6 100644 --- a/drivers/hv/ring_buffer.c +++ b/drivers/hv/ring_buffer.c @@ -107,35 +107,12 @@ static u32 hv_copyto_ringbuffer( return start_write_offset; } -/* - * - * hv_get_ringbuffer_availbytes() - * - * Get number of bytes available to read and to write to - * for the specified ring buffer - */ -static void -hv_get_ringbuffer_availbytes(const struct hv_ring_buffer_info *rbi, - u32 *read, u32 *write) -{ - u32 read_loc, write_loc, dsize; - - /* Capture the read/write indices before they changed */ - read_loc = READ_ONCE(rbi->ring_buffer->read_index); - write_loc = READ_ONCE(rbi->ring_buffer->write_index); - dsize = rbi->ring_datasize; - - *write = write_loc >= read_loc ? dsize - (write_loc - read_loc) : - read_loc - write_loc; - *read = dsize - *write; -} - /* Get various debug metrics for the specified ring buffer. */ int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info, struct hv_ring_buffer_debug_info *debug_info) { - u32 bytes_avail_towrite; - u32 bytes_avail_toread; + u32 read_index; + u32 write_index; mutex_lock(&ring_info->ring_buffer_mutex); @@ -144,13 +121,14 @@ int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info, return -EINVAL; } - hv_get_ringbuffer_availbytes(ring_info, - &bytes_avail_toread, - &bytes_avail_towrite); - debug_info->bytes_avail_toread = bytes_avail_toread; - debug_info->bytes_avail_towrite = bytes_avail_towrite; - debug_info->current_read_index = ring_info->ring_buffer->read_index; - debug_info->current_write_index = ring_info->ring_buffer->write_index; + read_index = READ_ONCE(ring_info->ring_buffer->read_index); + write_index = READ_ONCE(ring_info->ring_buffer->write_index); + debug_info->bytes_avail_toread = + hv_ringbuffer_avail_read(ring_info, read_index, write_index); + debug_info->bytes_avail_towrite = + hv_ringbuffer_avail_write(ring_info, read_index, write_index); + debug_info->current_read_index = read_index; + debug_info->current_write_index = write_index; debug_info->current_interrupt_mask = ring_info->ring_buffer->interrupt_mask; mutex_unlock(&ring_info->ring_buffer_mutex); @@ -282,8 +260,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel, read_index = READ_ONCE(outring_info->ring_buffer->read_index); old_write = READ_ONCE(outring_info->ring_buffer->write_index); - if (unlikely(read_index >= outring_info->ring_datasize || - old_write >= outring_info->ring_datasize)) { + if (unlikely(hv_ringbuffer_indices_invalid(outring_info, read_index, + old_write))) { spin_unlock_irqrestore(&outring_info->ring_lock, flags); pr_err_ratelimited("outbound ring indices out of range: relid %u read %u write %u size %u\n", channel->offermsg.child_relid, read_index, @@ -291,9 +269,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel, return -EIO; } - bytes_avail_towrite = old_write >= read_index ? - outring_info->ring_datasize - (old_write - read_index) : - read_index - old_write; + bytes_avail_towrite = hv_ringbuffer_avail_write(outring_info, read_index, + old_write); /* * If there is only room for the packet, assume it is full. @@ -445,10 +422,7 @@ static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi) */ write_loc = virt_load_acquire(&rbi->ring_buffer->write_index); - if (write_loc >= priv_read_loc) - return write_loc - priv_read_loc; - else - return (rbi->ring_datasize - priv_read_loc) + write_loc; + return hv_ringbuffer_avail_read(rbi, priv_read_loc, write_loc); } /* @@ -568,6 +542,7 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) { struct hv_ring_buffer_info *rbi = &channel->inbound; u32 curr_write_sz, pending_sz, bytes_read, start_read_index; + u32 write_index; /* * Make sure all reads are done before we update the read index since @@ -607,11 +582,13 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) return; /* - * Ensure the read of write_index in hv_get_bytes_to_write() - * happens after the read of pending_send_sz. + * Ensure the read of write_index happens after the read of + * pending_send_sz. */ virt_rmb(); - curr_write_sz = hv_get_bytes_to_write(rbi); + write_index = READ_ONCE(rbi->ring_buffer->write_index); + 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); /* @@ -627,8 +604,7 @@ void hv_pkt_iter_close(struct vmbus_channel *channel) * Exactly filling the ring buffer is treated as "not enough * space". The ring buffer always must have at least one byte * empty so the empty and full conditions are distinguishable. - * hv_get_bytes_to_write() doesn't fully tell the truth in - * this regard. + * curr_write_sz doesn't fully tell the truth in this regard. * * So first check if we were in the "enough free space" state * before we began the iteration. If so, the host was not diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h index 5c6582085b16..09f25ade88b0 100644 --- a/include/linux/hyperv.h +++ b/include/linux/hyperv.h @@ -209,31 +209,59 @@ struct hv_ring_buffer_info { }; +/* + * The indices live in memory shared with the untrusted host, so check a + * snapshot of them before using it as an offset or to compute a byte count. + */ +static inline bool +hv_ringbuffer_indices_invalid(const struct hv_ring_buffer_info *rbi, + u32 read_loc, u32 write_loc) +{ + return read_loc >= rbi->ring_datasize || + write_loc >= rbi->ring_datasize; +} + +/* + * Byte counts for a caller-supplied snapshot of the indices, so that the + * counts and the indices the caller goes on to use describe one state of the + * ring. + */ +static inline u32 +hv_ringbuffer_avail_write(const struct hv_ring_buffer_info *rbi, + u32 read_loc, u32 write_loc) +{ + u32 dsize = rbi->ring_datasize; + + return write_loc >= read_loc ? dsize - (write_loc - read_loc) : + read_loc - write_loc; +} + +static inline u32 +hv_ringbuffer_avail_read(const struct hv_ring_buffer_info *rbi, + u32 read_loc, u32 write_loc) +{ + return rbi->ring_datasize - + hv_ringbuffer_avail_write(rbi, read_loc, write_loc); +} + static inline u32 hv_get_bytes_to_read(const struct hv_ring_buffer_info *rbi) { - u32 read_loc, write_loc, dsize, read; + u32 read_loc, write_loc; - dsize = rbi->ring_datasize; read_loc = READ_ONCE(rbi->ring_buffer->read_index); write_loc = READ_ONCE(rbi->ring_buffer->write_index); - read = write_loc >= read_loc ? (write_loc - read_loc) : - (dsize - read_loc) + write_loc; - - return read; + return hv_ringbuffer_avail_read(rbi, read_loc, write_loc); } static inline u32 hv_get_bytes_to_write(const struct hv_ring_buffer_info *rbi) { - u32 read_loc, write_loc, dsize, write; + u32 read_loc, write_loc; - dsize = rbi->ring_datasize; read_loc = READ_ONCE(rbi->ring_buffer->read_index); write_loc = READ_ONCE(rbi->ring_buffer->write_index); - write = write_loc >= read_loc ? dsize - (write_loc - read_loc) : - read_loc - write_loc; - return write; + return hv_ringbuffer_avail_write(rbi, read_loc, write_loc); } static inline u32 hv_get_avail_to_write_percent(