[PATCH] stm class: Fix channel bitmap on 32-bit systems

[Alexander Shishkin]

Commit 7bd1d4093c2f ("stm class: Introduce an abstraction for System Trace
Module devices") naively calculates the channel bitmap size in 64-bit
chunks regardless of the size of underlying unsigned long, making the
bitmap half as big on a 32-bit system. This leads to an out of bounds
access with the upper half of the bitmap.

Fix this by using BITS_TO_LONGS. While at it, convert to using
struct_size() for the total size calculation of the master struct.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Fixes: 7bd1d4093c2f ("stm class: Introduce an abstraction for System Trace Module devices")
Reported-by: Mulu He <muluhe@xxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v4.4+
---
 drivers/hwtracing/stm/core.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/hwtracing/stm/core.c b/drivers/hwtracing/stm/core.c
index 5b5807cbcf7c..8c45e79e47db 100644
--- a/drivers/hwtracing/stm/core.c
+++ b/drivers/hwtracing/stm/core.c
@@ -166,11 +166,9 @@ stm_master(struct stm_device *stm, unsigned int idx)
 static int stp_master_alloc(struct stm_device *stm, unsigned int idx)
 {
 	struct stp_master *master;
-	size_t size;
 
-	size = ALIGN(stm->data->sw_nchannels, 8) / 8;
-	size += sizeof(struct stp_master);
-	master = kzalloc(size, GFP_ATOMIC);
+	master = kzalloc(struct_size(master, chan_map, BITS_TO_LONGS(stm->data->sw_nchannels)),
+			 GFP_ATOMIC);
 	if (!master)
 		return -ENOMEM;
 
-- 
2.20.1