linux-kernel - [PATCH AUTOSEL for 3.18 33/63] iommu/iova: Fix underflow bug in __alloc_and_insert_iova

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Message-ID: <20180303223228.27323-33-alexander.levin@microsoft.com>
Date:   Sat, 3 Mar 2018 22:33:29 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Nate Watterson <nwatters@...eaurora.org>,
        Joerg Roedel <jroedel@...e.de>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 3.18 33/63] iommu/iova: Fix underflow bug in
 __alloc_and_insert_iova_range

From: Nate Watterson <nwatters@...eaurora.org>

[ Upstream commit 5016bdb796b3726eec043ca0ce3be981f712c756 ]

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@...eaurora.org>
Signed-off-by: Joerg Roedel <jroedel@...e.de>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 drivers/iommu/iova.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index f6b17e6af2fb..c9e54a0b4bd1 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -118,7 +118,7 @@ static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
 				break;	/* found a free slot */
 		}
 adjust_limit_pfn:
-		limit_pfn = curr_iova->pfn_lo - 1;
+		limit_pfn = curr_iova->pfn_lo ? (curr_iova->pfn_lo - 1) : 0;
 move_left:
 		prev = curr;
 		curr = rb_prev(curr);
-- 
2.14.1