Commit message: xfs: don't BUG() on mixed direct and mapped I/O

We've had reports of generic/095 causing XFS to BUG() in
__xfs_get_blocks() due to the existence of delalloc blocks on a
direct I/O read. generic/095 issues a mix of various types of I/O,
including direct and memory mapped I/O to a single file. This is
clearly not supported behavior and is known to lead to such
problems. E.g., the lack of exclusion between the direct I/O and
write fault paths means that a write fault can allocate delalloc
blocks in a region of a file that was previously a hole after the
direct read has attempted to flush/inval the file range, but before
it actually reads the block mapping. In turn, the direct read
discovers a delalloc extent and cannot proceed.

While the appropriate solution here is to not mix direct and memory
mapped I/O to the same regions of the same file, the current
BUG_ON() behavior is probably overkill as it can crash the entire
system.  Instead, localize the failure to the I/O in question by
returning an error for a direct I/O that cannot be handled safely
due to delalloc blocks. Be careful to allow the case of a direct
write to post-eof delalloc blocks. This can occur due to speculative
preallocation and is safe as post-eof blocks are not accompanied by
dirty pages in pagecache (conversely, preallocation within eof must
have been zeroed, and thus dirtied, before the inode size could have
been increased beyond said blocks).

Finally, provide an additional warning if a direct I/O write occurs
while the file is memory mapped. This may not catch all problematic
scenarios, but provides a hint that some known-to-be-problematic I/O
methods are in use.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
CVE: CVE-2016-10741
CWE: CWE-362
Vulnerability description: In the Linux kernel before 4.9.3, fs/xfs/xfs_aops.c allows local users to cause a denial of service (system crash) because there is a race condition between direct and memory-mapped I/O (associated with a hole) that is handled with BUG_ON instead of an I/O failure.

Project: linux
File: xfs_aops.c

__xfs_get_blocks(
	struct inode		*inode,
	sector_t		iblock,
	struct buffer_head	*bh_result,
	int			create,
	bool			direct,
	bool			dax_fault)
{
	struct xfs_inode	*ip = XFS_I(inode);
	struct xfs_mount	*mp = ip->i_mount;
	xfs_fileoff_t		offset_fsb, end_fsb;
	int			error = 0;
	int			lockmode = 0;
	struct xfs_bmbt_irec	imap;
	int			nimaps = 1;
	xfs_off_t		offset;
	ssize_t			size;
	int			new = 0;
	bool			is_cow = false;
	bool			need_alloc = false;

	BUG_ON(create && !direct);

	if (XFS_FORCED_SHUTDOWN(mp))
		return -EIO;

	offset = (xfs_off_t)iblock << inode->i_blkbits;
	ASSERT(bh_result->b_size >= (1 << inode->i_blkbits));
	size = bh_result->b_size;

	if (!create && offset >= i_size_read(inode))
		return 0;

	/*
	 * Direct I/O is usually done on preallocated files, so try getting
	 * a block mapping without an exclusive lock first.
	 */
	lockmode = xfs_ilock_data_map_shared(ip);

	ASSERT(offset <= mp->m_super->s_maxbytes);
	if (offset + size > mp->m_super->s_maxbytes)
		size = mp->m_super->s_maxbytes - offset;
	end_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + size);
	offset_fsb = XFS_B_TO_FSBT(mp, offset);

	if (create && direct && xfs_is_reflink_inode(ip))
		is_cow = xfs_reflink_find_cow_mapping(ip, offset, &imap,
					&need_alloc);
	if (!is_cow) {
		error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb,
					&imap, &nimaps, XFS_BMAPI_ENTIRE);
		/*
		 * Truncate an overwrite extent if there's a pending CoW
		 * reservation before the end of this extent.  This
		 * forces us to come back to get_blocks to take care of
		 * the CoW.
		 */
		if (create && direct && nimaps &&
		    imap.br_startblock != HOLESTARTBLOCK &&
		    imap.br_startblock != DELAYSTARTBLOCK &&
		    !ISUNWRITTEN(&imap))
			xfs_reflink_trim_irec_to_next_cow(ip, offset_fsb,
					&imap);
	}
	ASSERT(!need_alloc);
 	if (error)
 		goto out_unlock;
 
 	/* for DAX, we convert unwritten extents directly */
 	if (create &&
 	    (!nimaps ||
	     (imap.br_startblock == HOLESTARTBLOCK ||
	      imap.br_startblock == DELAYSTARTBLOCK) ||
	     (IS_DAX(inode) && ISUNWRITTEN(&imap)))) {
		/*
		 * xfs_iomap_write_direct() expects the shared lock. It
		 * is unlocked on return.
		 */
		if (lockmode == XFS_ILOCK_EXCL)
			xfs_ilock_demote(ip, lockmode);

		error = xfs_iomap_write_direct(ip, offset, size,
					       &imap, nimaps);
		if (error)
			return error;
		new = 1;

		trace_xfs_get_blocks_alloc(ip, offset, size,
				ISUNWRITTEN(&imap) ? XFS_IO_UNWRITTEN
						   : XFS_IO_DELALLOC, &imap);
	} else if (nimaps) {
		trace_xfs_get_blocks_found(ip, offset, size,
				ISUNWRITTEN(&imap) ? XFS_IO_UNWRITTEN
						   : XFS_IO_OVERWRITE, &imap);
		xfs_iunlock(ip, lockmode);
	} else {
		trace_xfs_get_blocks_notfound(ip, offset, size);
		goto out_unlock;
	}

	if (IS_DAX(inode) && create) {
		ASSERT(!ISUNWRITTEN(&imap));
		/* zeroing is not needed at a higher layer */
		new = 0;
	}

	/* trim mapping down to size requested */
	xfs_map_trim_size(inode, iblock, bh_result, &imap, offset, size);

	/*
	 * For unwritten extents do not report a disk address in the buffered
	 * read case (treat as if we're reading into a hole).
	 */
	if (imap.br_startblock != HOLESTARTBLOCK &&
	    imap.br_startblock != DELAYSTARTBLOCK &&
	    (create || !ISUNWRITTEN(&imap))) {
		if (create && direct && !is_cow) {
			error = xfs_bounce_unaligned_dio_write(ip, offset_fsb,
					&imap);
			if (error)
				return error;
		}

		xfs_map_buffer(inode, bh_result, &imap, offset);
		if (ISUNWRITTEN(&imap))
			set_buffer_unwritten(bh_result);
		/* direct IO needs special help */
		if (create) {
			if (dax_fault)
				ASSERT(!ISUNWRITTEN(&imap));
			else
				xfs_map_direct(inode, bh_result, &imap, offset,
						is_cow);
		}
	}

	/*
	 * If this is a realtime file, data may be on a different device.
	 * to that pointed to from the buffer_head b_bdev currently.
	 */
	bh_result->b_bdev = xfs_find_bdev_for_inode(inode);

	/*
	 * If we previously allocated a block out beyond eof and we are now
	 * coming back to use it then we will need to flag it as new even if it
	 * has a disk address.
	 *
	 * With sub-block writes into unwritten extents we also need to mark
	 * the buffer as new so that the unwritten parts of the buffer gets
	 * correctly zeroed.
	 */
	if (create &&
	    ((!buffer_mapped(bh_result) && !buffer_uptodate(bh_result)) ||
	     (offset >= i_size_read(inode)) ||
 	     (new || ISUNWRITTEN(&imap))))
 		set_buffer_new(bh_result);
 
	BUG_ON(direct && imap.br_startblock == DELAYSTARTBLOCK);
 	return 0;
 
 out_unlock:
	xfs_iunlock(ip, lockmode);
	return error;
}