fat: fix buffer overflow in vfat_create_shortname()

When using the string representation of a random counter as part of the base
name, ensure that it is no longer than 4 bytes.

Since we are repeatedly decrementing the counter in a loop until we have found a
unique base name, the counter may wrap around zero; therefore, it is not enough
to mask its higher bits before entering the loop, this must be done inside the
loop.

[hirofumi@mail.parknet.co.jp: use snprintf()]
Signed-off-by: NNikolaus Schulz <microschulz@web.de>
Cc: stable@kernel.org
Signed-off-by: NOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/fat/namei_vfat.c

@@ -309,7 +309,7 @@ static int vfat_create_shortname(struct inode *dir, struct nls_table *nls,
 {
 	struct fat_mount_options *opts = &MSDOS_SB(dir->i_sb)->options;
 	wchar_t *ip, *ext_start, *end, *name_start;
-	unsigned char base[9], ext[4], buf[8], *p;
+	unsigned char base[9], ext[4], buf[5], *p;
 	unsigned char charbuf[NLS_MAX_CHARSET_SIZE];
 	int chl, chi;
 	int sz = 0, extlen, baselen, i, numtail_baselen, numtail2_baselen;
@@ -467,7 +467,7 @@ static int vfat_create_shortname(struct inode *dir, struct nls_table *nls,
 			return 0;
 	}
 
-	i = jiffies & 0xffff;
+	i = jiffies;
 	sz = (jiffies >> 16) & 0x7;
 	if (baselen > 2) {
 		baselen = numtail2_baselen;
@@ -476,7 +476,7 @@ static int vfat_create_shortname(struct inode *dir, struct nls_table *nls,
 	name_res[baselen + 4] = '~';
 	name_res[baselen + 5] = '1' + sz;
 	while (1) {
-		sprintf(buf, "%04X", i);
+		snprintf(buf, sizeof(buf), "%04X", i & 0xffff);
 		memcpy(&name_res[baselen], buf, 4);
 		if (vfat_find_form(dir, name_res) < 0)
 			break;