Commit message: ext4: allocate entire range in zero range

Currently there is a bug in zero range code which causes zero range
calls to only allocate block aligned portion of the range, while
ignoring the rest in some cases.

In some cases, namely if the end of the range is past i_size, we do
attempt to preallocate the last nonaligned block. However this might
cause kernel to BUG() in some carefully designed zero range requests
on setups where page size > block size.

Fix this problem by first preallocating the entire range, including
the nonaligned edges and converting the written extents to unwritten
in the next step. This approach will also give us the advantage of
having the range to be as linearly contiguous as possible.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
CVE: CVE-2015-0275
CWE: CWE-17
Vulnerability description: The ext4_zero_range function in fs/ext4/extents.c in the Linux kernel before 4.1 allows local users to cause a denial of service (BUG) via a crafted fallocate zero-range request.

Project: linux
File: extents.c

static long ext4_zero_range(struct file *file, loff_t offset,
			    loff_t len, int mode)
{
	struct inode *inode = file_inode(file);
	handle_t *handle = NULL;
	unsigned int max_blocks;
	loff_t new_size = 0;
	int ret = 0;
	int flags;
	int credits;
	int partial_begin, partial_end;
	loff_t start, end;
	ext4_lblk_t lblk;
	struct address_space *mapping = inode->i_mapping;
	unsigned int blkbits = inode->i_blkbits;

	trace_ext4_zero_range(inode, offset, len, mode);

	if (!S_ISREG(inode->i_mode))
		return -EINVAL;

	/* Call ext4_force_commit to flush all data in case of data=journal. */
	if (ext4_should_journal_data(inode)) {
		ret = ext4_force_commit(inode->i_sb);
		if (ret)
			return ret;
	}

	/*
	 * Write out all dirty pages to avoid race conditions
	 * Then release them.
	 */
	if (mapping->nrpages && mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
		ret = filemap_write_and_wait_range(mapping, offset,
						   offset + len - 1);
		if (ret)
			return ret;
	}

	/*
	 * Round up offset. This is not fallocate, we neet to zero out
	 * blocks, so convert interior block aligned part of the range to
	 * unwritten and possibly manually zero out unaligned parts of the
	 * range.
	 */
	start = round_up(offset, 1 << blkbits);
	end = round_down((offset + len), 1 << blkbits);

	if (start < offset || end > offset + len)
		return -EINVAL;
	partial_begin = offset & ((1 << blkbits) - 1);
	partial_end = (offset + len) & ((1 << blkbits) - 1);

	lblk = start >> blkbits;
	max_blocks = (end >> blkbits);
	if (max_blocks < lblk)
		max_blocks = 0;
 	else
 		max_blocks -= lblk;
 
 	mutex_lock(&inode->i_mutex);
 
 	/*
	 * Indirect files do not support unwritten extnets
	 */
	if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))) {
		ret = -EOPNOTSUPP;
		goto out_mutex;
	}

	if (!(mode & FALLOC_FL_KEEP_SIZE) &&
	     offset + len > i_size_read(inode)) {
		new_size = offset + len;
 		ret = inode_newsize_ok(inode, new_size);
 		if (ret)
 			goto out_mutex;
 	}
 
	flags = EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT;
	if (mode & FALLOC_FL_KEEP_SIZE)
		flags |= EXT4_GET_BLOCKS_KEEP_SIZE;

	/* Preallocate the range including the unaligned edges */
	if (partial_begin || partial_end) {
		ret = ext4_alloc_file_blocks(file,
				round_down(offset, 1 << blkbits) >> blkbits,
				(round_up((offset + len), 1 << blkbits) -
				 round_down(offset, 1 << blkbits)) >> blkbits,
				new_size, flags, mode);
		if (ret)
			goto out_mutex;

	}

	/* Zero range excluding the unaligned edges */
 	if (max_blocks > 0) {
		flags |= (EXT4_GET_BLOCKS_CONVERT_UNWRITTEN |
			  EXT4_EX_NOCACHE);
 
 		/* Now release the pages and zero block aligned part of pages*/
 		truncate_pagecache_range(inode, start, end - 1);
		inode->i_mtime = inode->i_ctime = ext4_current_time(inode);

		/* Wait all existing dio workers, newcomers will block on i_mutex */
		ext4_inode_block_unlocked_dio(inode);
		inode_dio_wait(inode);

		ret = ext4_alloc_file_blocks(file, lblk, max_blocks, new_size,
					     flags, mode);
		if (ret)
			goto out_dio;
		/*
		 * Remove entire range from the extent status tree.
		 *
		 * ext4_es_remove_extent(inode, lblk, max_blocks) is
		 * NOT sufficient.  I'm not sure why this is the case,
		 * but let's be conservative and remove the extent
		 * status tree for the entire inode.  There should be
		 * no outstanding delalloc extents thanks to the
		 * filemap_write_and_wait_range() call above.
		 */
		ret = ext4_es_remove_extent(inode, 0, EXT_MAX_BLOCKS);
		if (ret)
			goto out_dio;
	}
	if (!partial_begin && !partial_end)
		goto out_dio;

	/*
	 * In worst case we have to writeout two nonadjacent unwritten
	 * blocks and update the inode
	 */
	credits = (2 * ext4_ext_index_trans_blocks(inode, 2)) + 1;
	if (ext4_should_journal_data(inode))
		credits += 2;
	handle = ext4_journal_start(inode, EXT4_HT_MISC, credits);
	if (IS_ERR(handle)) {
		ret = PTR_ERR(handle);
		ext4_std_error(inode->i_sb, ret);
		goto out_dio;
	}

	inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
	if (new_size) {
		ext4_update_inode_size(inode, new_size);
	} else {
		/*
		* Mark that we allocate beyond EOF so the subsequent truncate
		* can proceed even if the new size is the same as i_size.
		*/
		if ((offset + len) > i_size_read(inode))
			ext4_set_inode_flag(inode, EXT4_INODE_EOFBLOCKS);
	}
	ext4_mark_inode_dirty(handle, inode);

	/* Zero out partial block at the edges of the range */
	ret = ext4_zero_partial_blocks(handle, inode, offset, len);

	if (file->f_flags & O_SYNC)
		ext4_handle_sync(handle);

	ext4_journal_stop(handle);
out_dio:
	ext4_inode_resume_unlocked_dio(inode);
out_mutex:
	mutex_unlock(&inode->i_mutex);
	return ret;
}