f2fs: sanity check of xattr entry size

There is a security report where f2fs_getxattr() has a hole to expose wrong
memory region when the image is malformed like this.

f2fs_getxattr: entry->e_name_len: 4, size: 12288, buffer_size: 16384, len: 4

Cc: <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/xattr.c

···
 static int lookup_all_xattrs(struct inode *inode, struct page *ipage,
 				unsigned int index, unsigned int len,
 				const char *name, struct f2fs_xattr_entry **xe,
-				void **base_addr)
+				void **base_addr, int *base_size)
 {
 	void *cur_addr, *txattr_addr, *last_addr = NULL;
 	nid_t xnid = F2FS_I(inode)->i_xattr_nid;
···
 	if (!size && !inline_size)
 		return -ENODATA;
 
-	txattr_addr = f2fs_kzalloc(F2FS_I_SB(inode),
-			inline_size + size + XATTR_PADDING_SIZE, GFP_NOFS);
+	*base_size = inline_size + size + XATTR_PADDING_SIZE;
+	txattr_addr = f2fs_kzalloc(F2FS_I_SB(inode), *base_size, GFP_NOFS);
 	if (!txattr_addr)
 		return -ENOMEM;
 
···
 
 		*xe = __find_inline_xattr(inode, txattr_addr, &last_addr,
 						index, len, name);
-		if (*xe)
+		if (*xe) {
+			*base_size = inline_size;
 			goto check;
+		}
 	}
 
 	/* read from xattr node block */
···
 	int error = 0;
 	unsigned int size, len;
 	void *base_addr = NULL;
+	int base_size;
 
 	if (name == NULL)
 		return -EINVAL;
···
 
 	down_read(&F2FS_I(inode)->i_xattr_sem);
 	error = lookup_all_xattrs(inode, ipage, index, len, name,
-				&entry, &base_addr);
+				&entry, &base_addr, &base_size);
 	up_read(&F2FS_I(inode)->i_xattr_sem);
 	if (error)
 		return error;
···
 
 	if (buffer) {
 		char *pval = entry->e_name + entry->e_name_len;
+
+		if (base_size - (pval - (char *)base_addr) < size) {
+			error = -ERANGE;
+			goto out;
+		}
 		memcpy(buffer, pval, size);
 	}
 	error = size;