linux-kernel - [PATCH] arm64: kvm: avoid overflow in integer division

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Message-Id: <20230517202352.793673-1-arnd@kernel.org>
Date:   Wed, 17 May 2023 22:23:39 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Shaoqin Huang <shahuang@...hat.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Gavin Shan <gshan@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>, James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: [PATCH] arm64: kvm: avoid overflow in integer division

From: Arnd Bergmann <arnd@...db.de>

The newly added kvm_mmu_split_nr_page_tables() function uses DIV_ROUND_DOWN_ULL()
to divide 64-bit addresses, but this requires a 32-bit divisior, and PUD_SIZE
may exceed that when 64KB pages are used:

arch/arm64/kvm/mmu.c: In function 'kvm_mmu_split_nr_page_tables':
include/linux/math.h:42:64: error: conversion from 'long unsigned int' to 'u32' {aka 'unsigned int'} changes value from '68719476736' to '0' [-Werror=overflow]
   42 |         DIV_ROUND_DOWN_ULL((unsigned long long)(ll) + (d) - 1, (d))
      |                                                                ^~~
include/linux/math.h:39:47: note: in definition of macro 'DIV_ROUND_DOWN_ULL'
   39 | #define DIV_ROUND_DOWN_ULL(ll, d) div_u64(ll, d)
      |                                               ^
arch/arm64/kvm/mmu.c:95:22: note: in expansion of macro 'DIV_ROUND_UP_ULL'
   95 |                 n += DIV_ROUND_UP_ULL(range, PUD_SIZE);
      |                      ^~~~~~~~~~~~~~~~

Since this code is only used on 64-bit targets, DIV_ROUND_UP() can deal with this
more easily, as it already takes 64-bit arguments.

Fixes: e7bf7a490c68 ("KVM: arm64: Split huge pages when dirty logging is enabled")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 arch/arm64/kvm/mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 3386bd28d267..6db9ef288ec3 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -92,8 +92,8 @@ static int kvm_mmu_split_nr_page_tables(u64 range)
 	int n = 0;
 
 	if (KVM_PGTABLE_MIN_BLOCK_LEVEL < 2)
-		n += DIV_ROUND_UP_ULL(range, PUD_SIZE);
-	n += DIV_ROUND_UP_ULL(range, PMD_SIZE);
+		n += DIV_ROUND_UP(range, PUD_SIZE);
+	n += DIV_ROUND_UP(range, PMD_SIZE);
 	return n;
 }
 
-- 
2.39.2