linux-kernel - [RFT][PATCH 1/7] powerpc/iommu: Avoid overflow at boundary

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Date:   Thu, 20 Aug 2020 16:19:50 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     mpe@...erman.id.au, benh@...nel.crashing.org, paulus@...ba.org
Cc:     sfr@...b.auug.org.au, hch@....de, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: [RFT][PATCH 1/7] powerpc/iommu: Avoid overflow at boundary_size

The boundary_size might be as large as ULONG_MAX, which means
that a device has no specific boundary limit. So either "+ 1"
or passing it to ALIGN() would potentially overflow.

According to kernel defines:
    #define ALIGN_MASK(x, mask) (((x) + (mask)) & ~(mask))
    #define ALIGN(x, a)	ALIGN_MASK(x, (typeof(x))(a) - 1)

We can simplify the logic here:
  ALIGN(boundary + 1, 1 << shift) >> shift
= ALIGN_MASK(b + 1, (1 << s) - 1) >> s
= {[b + 1 + (1 << s) - 1] & ~[(1 << s) - 1]} >> s
= [b + 1 + (1 << s) - 1] >> s
= [b + (1 << s)] >> s
= (b >> s) + 1

So fixing a potential overflow with the safer shortcut.

Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
Signed-off-by: Nicolin Chen <nicoleotsuka@...il.com>
Cc: Christoph Hellwig <hch@....de>
---
 arch/powerpc/kernel/iommu.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
index 9704f3f76e63..c01ccbf8afdd 100644
--- a/arch/powerpc/kernel/iommu.c
+++ b/arch/powerpc/kernel/iommu.c
@@ -236,15 +236,14 @@ static unsigned long iommu_range_alloc(struct device *dev,
 		}
 	}
 
-	if (dev)
-		boundary_size = ALIGN(dma_get_seg_boundary(dev) + 1,
-				      1 << tbl->it_page_shift);
-	else
-		boundary_size = ALIGN(1UL << 32, 1 << tbl->it_page_shift);
 	/* 4GB boundary for iseries_hv_alloc and iseries_hv_map */
+	boundary_size = dev ? dma_get_seg_boundary(dev) : U32_MAX;
+
+	/* Overflow-free shortcut for: ALIGN(b + 1, 1 << s) >> s */
+	boundary_size = (boundary_size >> tbl->it_page_shift) + 1;
 
 	n = iommu_area_alloc(tbl->it_map, limit, start, npages, tbl->it_offset,
-			     boundary_size >> tbl->it_page_shift, align_mask);
+			     boundary_size, align_mask);
 	if (n == -1) {
 		if (likely(pass == 0)) {
 			/* First try the pool from the start */
-- 
2.17.1