ext4: Fix integer overflow in ext4fs_read_symlink()

While zalloc() takes a size_t type, adding 1 to the le32 variable
will overflow.
A carefully crafted ext4 filesystem can exhibit an inode size of 0xffffffff
and as consequence zalloc() will do a zero allocation.

Later in the function the inode size is again used for copying data.
So an attacker can overwrite memory.

Avoid the overflow by using the __builtin_add_overflow() helper.

Signed-off-by: Richard Weinberger <richard@nod.at>

Author: richardweinberger

fs/ext4/ext4_common.c

@@ -2181,13 +2181,18 @@ static char *ext4fs_read_symlink(struct ext2fs_node *node)
 	struct ext2fs_node *diro = node;
 	int status;
 	loff_t actread;
+	size_t alloc_size;
 
 	if (!diro->inode_read) {
 		status = ext4fs_read_inode(diro->data, diro->ino, &diro->inode);
 		if (status == 0)
 			return NULL;
 	}
-	symlink = zalloc(le32_to_cpu(diro->inode.size) + 1);
+
+	if (__builtin_add_overflow(le32_to_cpu(diro->inode.size), 1, &alloc_size))
+		return NULL;
+
+	symlink = zalloc(alloc_size);
 	if (!symlink)
 		return NULL;