From 7a822dd050b165e6c2002fd059774de987410c08 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 4 Aug 2021 14:46:09 -0700 Subject: [PATCH] KVM: x86/mmu: Fix per-cpu counter corruption on 32-bit builds commit d5aaad6f83420efb8357ac8e11c868708b22d0a9 upstream. Take a signed 'long' instead of an 'unsigned long' for the number of pages to add/subtract to the total number of pages used by the MMU. This fixes a zero-extension bug on 32-bit kernels that effectively corrupts the per-cpu counter used by the shrinker. Per-cpu counters take a signed 64-bit value on both 32-bit and 64-bit kernels, whereas kvm_mod_used_mmu_pages() takes an unsigned long and thus an unsigned 32-bit value on 32-bit kernels. As a result, the value used to adjust the per-cpu counter is zero-extended (unsigned -> signed), not sign-extended (signed -> signed), and so KVM's intended -1 gets morphed to 4294967295 and effectively corrupts the counter. This was found by a staggering amount of sheer dumb luck when running kvm-unit-tests on a 32-bit KVM build. The shrinker just happened to kick in while running tests and do_shrink_slab() logged an error about trying to free a negative number of objects. The truly lucky part is that the kernel just happened to be a slightly stale build, as the shrinker no longer yells about negative objects as of commit 18bb473e5031 ("mm: vmscan: shrink deferred objects proportional to priority"). vmscan: shrink_slab: mmu_shrink_scan+0x0/0x210 [kvm] negative objects to delete nr=-858993460 Fixes: bc8a3d8925a8 ("kvm: mmu: Fix overflow on kvm mmu page limit calculation") Cc: stable@vger.kernel.org Cc: Ben Gardon Signed-off-by: Sean Christopherson Message-Id: <20210804214609.1096003-1-seanjc@google.com> Reviewed-by: Jim Mattson Signed-off-by: Paolo Bonzini Signed-off-by: Greg Kroah-Hartman --- arch/x86/kvm/mmu/mmu.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 716266ab177f4..ed469ddf20741 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -1546,7 +1546,7 @@ static int is_empty_shadow_page(u64 *spt) * aggregate version in order to make the slab shrinker * faster */ -static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, unsigned long nr) +static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, long nr) { kvm->arch.n_used_mmu_pages += nr; percpu_counter_add(&kvm_total_used_mmu_pages, nr);