This is a note to let you know that I've just added the patch titled Drivers: hv: vmbus: Prevent load re-ordering when reading ring buffer to the 4.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: drivers-hv-vmbus-prevent-load-re-ordering-when-readi.patch and it can be found in the queue-4.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit e4f172e56b8f200e2f04d6ccfa2543b68f6e829a Author: Michael Kelley <mikelley@xxxxxxxxxxxxx> Date: Sun Mar 27 08:25:10 2022 -0700 Drivers: hv: vmbus: Prevent load re-ordering when reading ring buffer [ Upstream commit b6cae15b5710c8097aad26a2e5e752c323ee5348 ] When reading a packet from a host-to-guest ring buffer, there is no memory barrier between reading the write index (to see if there is a packet to read) and reading the contents of the packet. The Hyper-V host uses store-release when updating the write index to ensure that writes of the packet data are completed first. On the guest side, the processor can reorder and read the packet data before the write index, and sometimes get stale packet data. Getting such stale packet data has been observed in a reproducible case in a VM on ARM64. Fix this by using virt_load_acquire() to read the write index, ensuring that reads of the packet data cannot be reordered before it. Preventing such reordering is logically correct, and with this change, getting stale data can no longer be reproduced. Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx> Reviewed-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx> Link: https://lore.kernel.org/r/1648394710-33480-1-git-send-email-mikelley@xxxxxxxxxxxxx Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c index 6cb45f256107..d97b30af9e03 100644 --- a/drivers/hv/ring_buffer.c +++ b/drivers/hv/ring_buffer.c @@ -365,7 +365,16 @@ int hv_ringbuffer_read(struct vmbus_channel *channel, static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi) { u32 priv_read_loc = rbi->priv_read_index; - u32 write_loc = READ_ONCE(rbi->ring_buffer->write_index); + u32 write_loc; + + /* + * The Hyper-V host writes the packet data, then uses + * store_release() to update the write_index. Use load_acquire() + * here to prevent loads of the packet data from being re-ordered + * before the read of the write_index and potentially getting + * stale data. + */ + write_loc = virt_load_acquire(&rbi->ring_buffer->write_index); if (write_loc >= priv_read_loc) return write_loc - priv_read_loc;