[PATCH] exfat: fix out-of-bounds in exfat_nls_to_ucs2()

[Jeongjun Park]

After the loop that converts characters to ucs2 ends, the variable i 
may be greater than or equal to len. However, when checking whether the
last byte of p_cstring is NULL, the variable i is used as is, resulting
in an out-of-bounds read if i >= len.

Therefore, to prevent this, we need to modify the function to check
whether i is less than len, and if i is greater than or equal to len,
to check p_cstring[len - 1] byte.

Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: syzbot+98cc76a76de46b3714d4@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
Fixes: 370e812b3ec1 ("exfat: add nls operations")
Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
---
 fs/exfat/nls.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/exfat/nls.c b/fs/exfat/nls.c
index 8243d94ceaf4..a52f3494eb20 100644
--- a/fs/exfat/nls.c
+++ b/fs/exfat/nls.c
@@ -616,7 +616,7 @@ static int exfat_nls_to_ucs2(struct super_block *sb,
 		unilen++;
 	}
 
-	if (p_cstring[i] != '\0')
+	if (p_cstring[min(i, len - 1)] != '\0')
 		lossy |= NLS_NAME_OVERLEN;
 
 	*uniname = '\0';
--