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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 I have started sending out backports for stable kernels up to 6.1. For 5.10 and 5.15 I have identified the required patches and backported them, but I haven't tested them yet. I'll get to testing and sending them out, but it will take a while; if anybody wants to help testing, I can provide my tentative patches. This is the list for 5.15: 27a59d57f073 KVM: x86/mmu: Use a bool for direct 86938ab6925b KVM: x86/mmu: Stop passing "direct" to mmu_alloc_root() 2e65e842c57d KVM: x86/mmu: Derive shadow MMU page role from parent 7f49777550e5 KVM: x86/mmu: Always pass 0 for @quadrant when gptes are 8 bytes 0cd8dc739833 KVM: x86/mmu: pull call to drop_large_spte() into __link_shadow_page() 0cb2af2ea66a KVM: x86: Fix shadow paging use-after-free due to unexpected GFN and the longer one for 5.10: b37233c911cb KVM: x86/mmu: Capture 'mmu' in a local variable when allocating roots ba0a194ffbfb KVM: x86/mmu: Allocate the lm_root before allocating PAE roots 748e52b9b736 KVM: x86/mmu: Allocate pae_root and lm_root pages in dedicated helper 6e6ec5848574 KVM: x86/mmu: Ensure MMU pages are available when allocating roots 27a59d57f073 KVM: x86/mmu: Use a bool for direct 86938ab6925b KVM: x86/mmu: Stop passing "direct" to mmu_alloc_root() 03fffc5493c8 KVM: x86/mmu: Refactor shadow walk in __direct_map() to reduce indentation f81602958c11 KVM: X86: Fix missed remote tlb flush in rmap_write_protect() 65855ed8b034 KVM: X86: Synchronize the shadow pagetable before link it 2e65e842c57d KVM: x86/mmu: Derive shadow MMU page role from parent 7f49777550e5 KVM: x86/mmu: Always pass 0 for @quadrant when gptes are 8 bytes 6e0918aec49a KVM: x86/mmu: Check PDPTRs before allocating PAE roots 0cd8dc739833 KVM: x86/mmu: pull call to drop_large_spte() into __link_shadow_page() 0cb2af2ea66a KVM: x86: Fix shadow paging use-after-free due to unexpected GFN Paolo On Sun, May 3, 2026 at 10:10 PM Paolo Bonzini wrote: > > From: Sean Christopherson > > The shadow MMU computes GFNs for direct shadow pages using sp->gfn plus > the SPTE index. This assumption breaks for shadow paging if the guest > page tables are modified between VM entries (similar to commit > aad885e77496, "KVM: x86/mmu: Drop/zap existing present SPTE even > when creating an MMIO SPTE", 2026-03-27). The flow is as follows: > > - a PDE is installed for a 2MB mapping, and a page in that area is > accessed. KVM creates a kvm_mmu_page consisting of 512 4KB pages; > the kvm_mmu_page is marked by FNAME(fetch) as direct-mapped because > the guest's mapping is a huge page (and thus contiguous). > > - the PDE mapping is changed from outside the guest. > > - the guest accesses another page in the same 2MB area. KVM installs > a new leaf SPTE and rmap entry; the SPTE uses the "correct" GFN > (i.e. based on the new mapping, as changed in the previous step) but > that GFN is outside of the [sp->gfn, sp->gfn + 511] range; therefore > the rmap entry cannot be found and removed when the kvm_mmu_page > is zapped. > > - the memslot that covers the first 2MB mapping is deleted, and the > kvm_mmu_page for the now-invalid GPA is zapped. However, rmap_remove() > only looks at the [sp->gfn, sp->gfn + 511] range established in step 1, > and fails to find the rmap entry that was recorded by step 3. > > - any operation that causes an rmap walk for the same page accessed > by step 3 then walks a stale rmap and dereferences a freed kvm_mmu_age. > This includes dirty logging or MMU notifier invalidations (e.g., from > MADV_DONTNEED). > > The underlying issue is that KVM's walking of shadow PTEs assumes that > if a SPTE is present when KVM wants to install a non-leaf SPTE, then the > existing kvm_mmu_page must be for the correct gfn. Because the only way > for the gfn to be wrong is if KVM messed up and failed to zap a SPTE... > which shouldn't happen, but *actually* only happens in response to a > guest write. > > That bug dates back literally forever, as even the first version of KVM > assumes that the GFN matches and walks into the "wrong" shadow page. > However, that was only an imprecision until 2032a93d66fa ("KVM: MMU: > Don't allocate gfns page for direct mmu pages") came along. > > Fix it by checking for a target gfn mismatch and zapping the existing > SPTE. That way the old SP and rmap entries are gone, KVM installs > the rmap in the right location, and everyone is happy. > > Fixes: 2032a93d66fa ("KVM: MMU: Don't allocate gfns page for direct mmu pages") > Fixes: 6aa8b732ca01 ("kvm: userspace interface") > Reported-by: Alexander Bulekov > Reported-by: Fred Griffoul > Cc: stable@vger.kernel.org > Signed-off-by: Sean Christopherson > Signed-off-by: Paolo Bonzini > --- > arch/x86/kvm/mmu/mmu.c | 35 ++++++++++++++--------------------- > 1 file changed, 14 insertions(+), 21 deletions(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 24fbc9ea502a..892246204435 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -182,6 +182,8 @@ static struct kmem_cache *pte_list_desc_cache; > struct kmem_cache *mmu_page_header_cache; > > static void mmu_spte_set(u64 *sptep, u64 spte); > +static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp, > + u64 *spte, struct list_head *invalid_list); > > struct kvm_mmu_role_regs { > const unsigned long cr0; > @@ -1287,19 +1289,6 @@ static void drop_spte(struct kvm *kvm, u64 *sptep) > rmap_remove(kvm, sptep); > } > > -static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush) > -{ > - struct kvm_mmu_page *sp; > - > - sp = sptep_to_sp(sptep); > - WARN_ON_ONCE(sp->role.level == PG_LEVEL_4K); > - > - drop_spte(kvm, sptep); > - > - if (flush) > - kvm_flush_remote_tlbs_sptep(kvm, sptep); > -} > - > /* > * Write-protect on the specified @sptep, @pt_protect indicates whether > * spte write-protection is caused by protecting shadow page table. > @@ -2466,7 +2455,8 @@ static struct kvm_mmu_page *kvm_mmu_get_child_sp(struct kvm_vcpu *vcpu, > { > union kvm_mmu_page_role role; > > - if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) > + if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep) && > + spte_to_child_sp(*sptep) && spte_to_child_sp(*sptep)->gfn == gfn) > return ERR_PTR(-EEXIST); > > role = kvm_mmu_child_role(sptep, direct, access); > @@ -2544,13 +2534,16 @@ static void __link_shadow_page(struct kvm *kvm, > > BUILD_BUG_ON(VMX_EPT_WRITABLE_MASK != PT_WRITABLE_MASK); > > - /* > - * If an SPTE is present already, it must be a leaf and therefore > - * a large one. Drop it, and flush the TLB if needed, before > - * installing sp. > - */ > - if (is_shadow_present_pte(*sptep)) > - drop_large_spte(kvm, sptep, flush); > + if (is_shadow_present_pte(*sptep)) { > + struct kvm_mmu_page *parent_sp; > + LIST_HEAD(invalid_list); > + > + parent_sp = sptep_to_sp(sptep); > + WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K); > + > + mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list); > + kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true); > + } > > spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp)); > > -- > 2.54.0