Re: [PATCH v5 4/4] KVM: mmu: remove over-aggressive warnings
From: Sean Christopherson <seanjc@google.com>
Date: 2021-12-30 19:22:11
Also in:
kvm, kvmarm, lkml
Subsystem:
kernel virtual machine (kvm), the rest · Maintainers:
Paolo Bonzini, Linus Torvalds
On Mon, Nov 29, 2021, David Stevens wrote:
quoted hunk ↗ jump to hunk
From: David Stevens <redacted> Remove two warnings that require ref counts for pages to be non-zero, as mapped pfns from follow_pfn may not have an initialized ref count. Signed-off-by: David Stevens <redacted> --- arch/x86/kvm/mmu/mmu.c | 7 ------- virt/kvm/kvm_main.c | 2 +- 2 files changed, 1 insertion(+), 8 deletions(-)diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 0626395ff1d9..7c4c7fededf0 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c@@ -621,13 +621,6 @@ static int mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep) pfn = spte_to_pfn(old_spte); - /* - * KVM does not hold the refcount of the page used by - * kvm mmu, before reclaiming the page, we should - * unmap it from mmu first. - */ - WARN_ON(!kvm_is_reserved_pfn(pfn) && !page_count(pfn_to_page(pfn))); - if (is_accessed_spte(old_spte)) kvm_set_pfn_accessed(pfn);diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 16a8a71f20bf..d81edcb3e107 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c@@ -170,7 +170,7 @@ bool kvm_is_zone_device_pfn(kvm_pfn_t pfn) * the device has been pinned, e.g. by get_user_pages(). WARN if the * page_count() is zero to help detect bad usage of this helper.
Stale comment.
*/ - if (!pfn_valid(pfn) || WARN_ON_ONCE(!page_count(pfn_to_page(pfn)))) + if (!pfn_valid(pfn) || !page_count(pfn_to_page(pfn)))
Hrm, I know the whole point of this series is to support pages without an elevated refcount, but this WARN was extremely helpful in catching several use-after-free bugs in the TDP MMU. We talked about burying a slow check behind MMU_WARN_ON, but that isn't very helpful because no one runs with MMU_WARN_ON, and this is also a type of check that's most useful if it runs in production. IIUC, this series explicitly disallows using pfns that have a struct page without refcounting, and the issue with the WARN here is that kvm_is_zone_device_pfn() is called by kvm_is_reserved_pfn() before ensure_pfn_ref() rejects problematic pages, i.e. triggers false positive. So, can't we preserve the use-after-free benefits of the check by moving it to where KVM releases the PFN? I.e.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index fbca2e232e94..675b835525fa 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c@@ -2904,15 +2904,19 @@ EXPORT_SYMBOL_GPL(kvm_release_pfn_dirty); void kvm_set_pfn_dirty(kvm_pfn_t pfn) { - if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn)) + if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn)) { + WARN_ON_ONCE(!page_count(pfn_to_page(pfn))); SetPageDirty(pfn_to_page(pfn)); + } } EXPORT_SYMBOL_GPL(kvm_set_pfn_dirty); void kvm_set_pfn_accessed(kvm_pfn_t pfn) { - if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn)) + if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn)) { + WARN_ON_ONCE(!page_count(pfn_to_page(pfn))); mark_page_accessed(pfn_to_page(pfn)); + } } EXPORT_SYMBOL_GPL(kvm_set_pfn_accessed);
In a way, that's even better than the current check as it makes it more obvious that the WARN is due to a use-after-free. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel