iommu/iova: Fix underflow bug in __alloc_and_insert_iova_range

iommu/iova: Fix underflow bug in __alloc_and_insert_iova_range

Normally, calling alloc_iova() using an iova_domain with insufficient
pfns remaining between start_pfn and dma_limit will fail and return a
NULL pointer. Unexpectedly, if such a "full" iova_domain contains an
iova with pfn_lo == 0, the alloc_iova() call will instead succeed and
return an iova containing invalid pfns.

This is caused by an underflow bug in __alloc_and_insert_iova_range()
that occurs after walking the "full" iova tree when the search ends
at the iova with pfn_lo == 0 and limit_pfn is then adjusted to be just
below that (-1). This (now huge) limit_pfn gives the impression that a
vast amount of space is available between it and start_pfn and thus
a new iova is allocated with the invalid pfn_hi value, 0xFFF.... .

To rememdy this, a check is introduced to ensure that adjustments to
limit_pfn will not underflow.

This issue has been observed in the wild, and is easily reproduced with
the following sample code.

	struct iova_domain *iovad = kzalloc(sizeof(*iovad), GFP_KERNEL);
	struct iova *rsvd_iova, *good_iova, *bad_iova;
	unsigned long limit_pfn = 3;
	unsigned long start_pfn = 1;
	unsigned long va_size = 2;

	init_iova_domain(iovad, SZ_4K, start_pfn, limit_pfn);
	rsvd_iova = reserve_iova(iovad, 0, 0);
	good_iova = alloc_iova(iovad, va_size, limit_pfn, true);
	bad_iova = alloc_iova(iovad, va_size, limit_pfn, true);

Prior to the patch, this yielded:
	*rsvd_iova == {0, 0}   /* Expected */
	*good_iova == {2, 3}   /* Expected */
	*bad_iova  == {-2, -1} /* Oh no... */

After the patch, bad_iova is NULL as expected since inadequate
space remains between limit_pfn and start_pfn after allocating
good_iova.

Signed-off-by: Nate Watterson <nwatters@codeaurora.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>