[1/3] KVM: x86: Avoid overflowing nr_mmu_pages in kvm_arch_commit_memory_region()

Commit Message

Maciej S. Szmigiero Nov. 26, 2021, 12:31 a.m. UTC

From: "Maciej S. Szmigiero" <[e-mail]>

With kvm->nr_memslot_pages capped at ULONG_MAX we can't safely multiply it
by KVM_PERMILLE_MMU_PAGES (20) since this operation can possibly overflow
an unsigned long variable.

Rewrite this "* 20 / 1000" operation as "/ 50" instead to avoid such
overflow.

Signed-off-by: Maciej S. Szmigiero <[e-mail]>
---
 arch/x86/include/asm/kvm_host.h | 2 +-
 arch/x86/kvm/x86.c              | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

Comments

Sean Christopherson Nov. 29, 2021, 9:03 p.m. UTC | #1

On Fri, Nov 26, 2021, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <[e-mail]>
> 
> With kvm->nr_memslot_pages capped at ULONG_MAX we can't safely multiply it
> by KVM_PERMILLE_MMU_PAGES (20) since this operation can possibly overflow
> an unsigned long variable.
> 
> Rewrite this "* 20 / 1000" operation as "/ 50" instead to avoid such
> overflow.
> 
> Signed-off-by: Maciej S. Szmigiero <[e-mail]>
> ---

Reviewed-by: Sean Christopherson <[e-mail]>

Patch

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 1fcb345bc107..8cd1d254c948 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -135,7 +135,7 @@ 
 #define KVM_HPAGE_MASK(x)	(~(KVM_HPAGE_SIZE(x) - 1))
 #define KVM_PAGES_PER_HPAGE(x)	(KVM_HPAGE_SIZE(x) / PAGE_SIZE)
 
-#define KVM_PERMILLE_MMU_PAGES 20
+#define KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO 50
 #define KVM_MIN_ALLOC_MMU_PAGES 64UL
 #define KVM_MMU_HASH_SHIFT 12
 #define KVM_NUM_MMU_PAGES (1 << KVM_MMU_HASH_SHIFT)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 04e8dabc187d..69330b395f12 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11753,8 +11753,7 @@  void kvm_arch_commit_memory_region(struct kvm *kvm,
 	    (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
 		unsigned long nr_mmu_pages;
 
-		nr_mmu_pages = kvm->nr_memslot_pages * KVM_PERMILLE_MMU_PAGES;
-		nr_mmu_pages /= 1000;
+		nr_mmu_pages = kvm->nr_memslot_pages / KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO;
 		nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
 		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
 	}