exfat: fix out-of-bounds in exfat_nls_to_ucs2()

Since the len argument value passed to exfat_ioctl_set_volume_label()
from exfat_nls_to_utf16() is passed 1 too large, an out-of-bounds read
occurs when dereferencing p_cstring in exfat_nls_to_ucs2() later.

And because of the NLS_NAME_OVERLEN macro, another error occurs when
creating a file with a period at the end using utf8 and other iocharsets.

So to avoid this, you should remove the code that uses NLS_NAME_OVERLEN
macro and make the len argument value be the length of the label string,
but with a maximum length of FSLABEL_MAX - 1.

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
Fixes: d01579d590f7 ("exfat: Add support for FS_IOC_{GET,SET}FSLABEL")
Suggested-by: Pali Rohár <[e-mail]>
Signed-off-by: Jeongjun Park <[e-mail]>
Signed-off-by: Namjae Jeon <[e-mail]>

fs/exfat/exfat_fs.h

···
 enum {
 	NLS_NAME_NO_LOSSY =	0,	/* no lossy */
 	NLS_NAME_LOSSY =	1 << 0,	/* just detected incorrect filename(s) */
-	NLS_NAME_OVERLEN =	1 << 1,	/* the length is over than its limit */
 };
 
 #define EXFAT_HASH_BITS		8

fs/exfat/file.c

···
 static int exfat_ioctl_set_volume_label(struct super_block *sb,
 					unsigned long arg)
 {
-	int ret = 0, lossy;
-	char label[FSLABEL_MAX];
+	int ret = 0, lossy, label_len;
+	char label[FSLABEL_MAX] = {0};
 	struct exfat_uni_name uniname;
 
 	if (!capable(CAP_SYS_ADMIN))
···
 		return -EFAULT;
 
 	memset(&uniname, 0, sizeof(uniname));
+	label_len = strnlen(label, FSLABEL_MAX - 1);
 	if (label[0]) {
-		ret = exfat_nls_to_utf16(sb, label, FSLABEL_MAX,
+		ret = exfat_nls_to_utf16(sb, label, label_len,
 					 &uniname, &lossy);
 		if (ret < 0)
 			return ret;

fs/exfat/namei.c

···
 		return namelen; /* return error value */
 
 	if ((lossy && !lookup) || !namelen)
-		return (lossy & NLS_NAME_OVERLEN) ? -ENAMETOOLONG : -EINVAL;
+		return -EINVAL;
 
 	return 0;
 }

fs/exfat/nls.c

···
 		unilen++;
 	}
 
-	if (p_cstring[i] != '\0')
-		lossy |= NLS_NAME_OVERLEN;
-
 	*uniname = '\0';
 	p_uniname->name_len = unilen;
 	p_uniname->name_hash = exfat_calc_chksum16(upname, unilen << 1, 0,