[PATCH] hyper-v: Check for ring buffer in hv_get_bytes_to_read/write

[Mohammed Gamal <mgamal@redhat.com>]

This patch adds a check for the presence of the ring buffer in
hv_get_bytes_to_read/write() to avoid possible NULL pointer dereferences.
If the ring buffer is not yet allocated, return 0 bytes to be read/written.

The root cause is that code that accesses the ring buffer including
hv_get_bytes_to_read/write() could be vulnerable to the race condition
discussed in https://lkml.org/lkml/2018/10/18/779

This race is being addressed by the patch series by Kimberly Brown in
https://lkml.org/lkml/2019/2/21/1236 which is not final yet

Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
---
 include/linux/hyperv.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 64698ec8f2ac..7b2f566250b2 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -148,6 +148,9 @@ static inline u32 hv_get_bytes_to_read(const struct hv_ring_buffer_info *rbi)
 {
 	u32 read_loc, write_loc, dsize, read;
 
+	if (!rbi->ring_buffer)
+		return 0;
+
 	dsize = rbi->ring_datasize;
 	read_loc = rbi->ring_buffer->read_index;
 	write_loc = READ_ONCE(rbi->ring_buffer->write_index);
@@ -162,6 +165,9 @@ static inline u32 hv_get_bytes_to_write(const struct hv_ring_buffer_info *rbi)
 {
 	u32 read_loc, write_loc, dsize, write;
 
+	if (!rbi->ring_buffer)
+		return 0;
+
 	dsize = rbi->ring_datasize;
 	read_loc = READ_ONCE(rbi->ring_buffer->read_index);
 	write_loc = rbi->ring_buffer->write_index;
-- 
2.18.1