staging: erofs: keep corrupted fs from crashing kernel in erofs_readdir()

After commit 419d6efc50e9, kernel cannot be crashed in the namei
path. However, corrupted nameoff can do harm in the process of
readdir for scenerios without dm-verity as well. Fix it now.

Fixes: 3aa8ec716e52 ("staging: erofs: add directory operations")
Cc: <[e-mail]> # 4.19+
Signed-off-by: Gao Xiang <[e-mail]>
Reviewed-by: Chao Yu <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/erofs/dir.c

···
 	[EROFS_FT_SYMLINK]	= DT_LNK,
 };
 
+static void debug_one_dentry(unsigned char d_type, const char *de_name,
+			     unsigned int de_namelen)
+{
+#ifdef CONFIG_EROFS_FS_DEBUG
+	/* since the on-disk name could not have the trailing '\0' */
+	unsigned char dbg_namebuf[EROFS_NAME_LEN + 1];
+
+	memcpy(dbg_namebuf, de_name, de_namelen);
+	dbg_namebuf[de_namelen] = '\0';
+
+	debugln("found dirent %s de_len %u d_type %d", dbg_namebuf,
+		de_namelen, d_type);
+#endif
+}
+
 static int erofs_fill_dentries(struct dir_context *ctx,
 			       void *dentry_blk, unsigned int *ofs,
 			       unsigned int nameoff, unsigned int maxsize)
···
 	de = dentry_blk + *ofs;
 	while (de < end) {
 		const char *de_name;
-		int de_namelen;
+		unsigned int de_namelen;
 		unsigned char d_type;
-#ifdef CONFIG_EROFS_FS_DEBUG
-		unsigned int dbg_namelen;
-		unsigned char dbg_namebuf[EROFS_NAME_LEN];
-#endif
 
-		if (unlikely(de->file_type < EROFS_FT_MAX))
+		if (de->file_type < EROFS_FT_MAX)
 			d_type = erofs_filetype_table[de->file_type];
 		else
 			d_type = DT_UNKNOWN;
···
 		nameoff = le16_to_cpu(de->nameoff);
 		de_name = (char *)dentry_blk + nameoff;
 
-		de_namelen = unlikely(de + 1 >= end) ?
-			/* last directory entry */
-			strnlen(de_name, maxsize - nameoff) :
-			le16_to_cpu(de[1].nameoff) - nameoff;
+		/* the last dirent in the block? */
+		if (de + 1 >= end)
+			de_namelen = strnlen(de_name, maxsize - nameoff);
+		else
+			de_namelen = le16_to_cpu(de[1].nameoff) - nameoff;
 
 		/* a corrupted entry is found */
-		if (unlikely(de_namelen < 0)) {
+		if (unlikely(nameoff + de_namelen > maxsize ||
+			     de_namelen > EROFS_NAME_LEN)) {
 			DBG_BUGON(1);
 			return -EIO;
 		}
 
-#ifdef CONFIG_EROFS_FS_DEBUG
-		dbg_namelen = min(EROFS_NAME_LEN - 1, de_namelen);
-		memcpy(dbg_namebuf, de_name, dbg_namelen);
-		dbg_namebuf[dbg_namelen] = '\0';
-
-		debugln("%s, found de_name %s de_len %d d_type %d", __func__,
-			dbg_namebuf, de_namelen, d_type);
-#endif
-
+		debug_one_dentry(d_type, de_name, de_namelen);
 		if (!dir_emit(ctx, de_name, de_namelen,
 			      le64_to_cpu(de->nid), d_type))
 			/* stopped by some reason */