[PATCH] f2fs: prevent possible int overflow in dir_block_index()

[Nikita Zhandarovich]

The result of multiplication between values derived from functions
dir_buckets() and bucket_blocks() *could* technically reach
2^30 * 2^2 = 2^32.

While unlikely to happen, it is prudent to ensure that it will not
lead to integer overflow. Thus, use mul_u32_u32() as it's more
appropriate to mitigate the issue.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 3843154598a0 ("f2fs: introduce large directory support")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@xxxxxxxxxx>
---
 fs/f2fs/dir.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index cbd7a5e96a37..14900ca8a9ff 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -166,7 +166,8 @@ static unsigned long dir_block_index(unsigned int level,
 	unsigned long bidx = 0;
 
 	for (i = 0; i < level; i++)
-		bidx += dir_buckets(i, dir_level) * bucket_blocks(i);
+		bidx += mul_u32_u32(dir_buckets(i, dir_level),
+				    bucket_blocks(i));
 	bidx += idx * bucket_blocks(level);
 	return bidx;
 }