Commit message: xfs: don't call xfs_da_shrink_inode with NULL bp

xfs_attr3_leaf_create may have errored out before instantiating a buffer,
for example if the blkno is out of range.  In that case there is no work
to do to remove it, and in fact xfs_da_shrink_inode will lead to an oops
if we try.

This also seems to fix a flaw where the original error from
xfs_attr3_leaf_create gets overwritten in the cleanup case, and it
removes a pointless assignment to bp which isn't used after this.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=199969
Reported-by: Xu, Wen <wen.xu@gatech.edu>
Tested-by: Xu, Wen <wen.xu@gatech.edu>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
CVE: CVE-2018-13094
CWE: CWE-476
Vulnerability description: An issue was discovered in fs/xfs/libxfs/xfs_attr_leaf.c in the Linux kernel through 4.17.3. An OOPS may occur for a corrupted xfs image after xfs_da_shrink_inode() is called with a NULL bp.

Project: linux
File: xfs_attr_leaf.c

xfs_attr_shortform_to_leaf(
	struct xfs_da_args	*args,
	struct xfs_buf		**leaf_bp)
{
	xfs_inode_t *dp;
	xfs_attr_shortform_t *sf;
	xfs_attr_sf_entry_t *sfe;
	xfs_da_args_t nargs;
	char *tmpbuffer;
	int error, i, size;
	xfs_dablk_t blkno;
	struct xfs_buf *bp;
	xfs_ifork_t *ifp;

	trace_xfs_attr_sf_to_leaf(args);

	dp = args->dp;
	ifp = dp->i_afp;
	sf = (xfs_attr_shortform_t *)ifp->if_u1.if_data;
	size = be16_to_cpu(sf->hdr.totsize);
	tmpbuffer = kmem_alloc(size, KM_SLEEP);
	ASSERT(tmpbuffer != NULL);
	memcpy(tmpbuffer, ifp->if_u1.if_data, size);
	sf = (xfs_attr_shortform_t *)tmpbuffer;

	xfs_idata_realloc(dp, -size, XFS_ATTR_FORK);
	xfs_bmap_local_to_extents_empty(dp, XFS_ATTR_FORK);

	bp = NULL;
	error = xfs_da_grow_inode(args, &blkno);
	if (error) {
		/*
		 * If we hit an IO error middle of the transaction inside
		 * grow_inode(), we may have inconsistent data. Bail out.
		 */
		if (error == -EIO)
			goto out;
		xfs_idata_realloc(dp, size, XFS_ATTR_FORK);	/* try to put */
		memcpy(ifp->if_u1.if_data, tmpbuffer, size);	/* it back */
		goto out;
	}

 	ASSERT(blkno == 0);
 	error = xfs_attr3_leaf_create(args, blkno, &bp);
 	if (error) {
		error = xfs_da_shrink_inode(args, 0, bp);
		bp = NULL;
		if (error)
 			goto out;
 		xfs_idata_realloc(dp, size, XFS_ATTR_FORK);	/* try to put */
 		memcpy(ifp->if_u1.if_data, tmpbuffer, size);	/* it back */
		goto out;
	}

	memset((char *)&nargs, 0, sizeof(nargs));
	nargs.dp = dp;
	nargs.geo = args->geo;
	nargs.firstblock = args->firstblock;
	nargs.dfops = args->dfops;
	nargs.total = args->total;
	nargs.whichfork = XFS_ATTR_FORK;
	nargs.trans = args->trans;
	nargs.op_flags = XFS_DA_OP_OKNOENT;

	sfe = &sf->list[0];
	for (i = 0; i < sf->hdr.count; i++) {
		nargs.name = sfe->nameval;
		nargs.namelen = sfe->namelen;
		nargs.value = &sfe->nameval[nargs.namelen];
		nargs.valuelen = sfe->valuelen;
		nargs.hashval = xfs_da_hashname(sfe->nameval,
						sfe->namelen);
		nargs.flags = XFS_ATTR_NSP_ONDISK_TO_ARGS(sfe->flags);
		error = xfs_attr3_leaf_lookup_int(bp, &nargs); /* set a->index */
		ASSERT(error == -ENOATTR);
		error = xfs_attr3_leaf_add(bp, &nargs);
		ASSERT(error != -ENOSPC);
		if (error)
			goto out;
		sfe = XFS_ATTR_SF_NEXTENTRY(sfe);
	}
	error = 0;
	*leaf_bp = bp;
out:
	kmem_free(tmpbuffer);
	return error;
}