linux-kernel - [RESEND][PATCH 6/7] x86/amd_gart: Avoid overflow at boundary

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Message-Id: <20200831203811.8494-7-nicoleotsuka@gmail.com>
Date:   Mon, 31 Aug 2020 13:38:10 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     mpe@...erman.id.au, benh@...nel.crashing.org, paulus@...ba.org,
        rth@...ddle.net, ink@...assic.park.msu.ru, mattst88@...il.com,
        tony.luck@...el.com, fenghua.yu@...el.com, schnelle@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, borntraeger@...ibm.com, davem@...emloft.net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, James.Bottomley@...senPartnership.com, deller@....de
Cc:     sfr@...b.auug.org.au, hch@....de, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: [RESEND][PATCH 6/7] x86/amd_gart: 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.

Signed-off-by: Nicolin Chen <nicoleotsuka@...il.com>
Cc: Christoph Hellwig <hch@....de>
---
 arch/x86/kernel/amd_gart_64.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/amd_gart_64.c b/arch/x86/kernel/amd_gart_64.c
index e89031e9c847..7fa0bb490065 100644
--- a/arch/x86/kernel/amd_gart_64.c
+++ b/arch/x86/kernel/amd_gart_64.c
@@ -96,8 +96,8 @@ static unsigned long alloc_iommu(struct device *dev, int size,
 
 	base_index = ALIGN(iommu_bus_base & dma_get_seg_boundary(dev),
 			   PAGE_SIZE) >> PAGE_SHIFT;
-	boundary_size = ALIGN((u64)dma_get_seg_boundary(dev) + 1,
-			      PAGE_SIZE) >> PAGE_SHIFT;
+	/* Overflow-free shortcut for: ALIGN(b + 1, 1 << s) >> s */
+	boundary_size = (dma_get_seg_boundary(dev) >> PAGE_SHIFT) + 1;
 
 	spin_lock_irqsave(&iommu_bitmap_lock, flags);
 	offset = iommu_area_alloc(iommu_gart_bitmap, iommu_pages, next_bit,
-- 
2.17.1