Read mmu_invalidate_seq before dropping the mmap_lock so that KVM can detect if the results of vma_lookup() (e.g. vma_shift) become stale before it acquires kvm->mmu_lock. This fixes a theoretical bug where a VMA could be changed by userspace after vma_lookup() and before KVM reads the mmu_invalidate_seq, causing KVM to install page table entries based on a (possibly) no-longer-valid vma_shift.

Re-order the MMU cache top-up to earlier in user_mem_abort() so that it is not done after KVM has read mmu_invalidate_seq (i.e. so as to avoid inducing spurious fault retries).

This bug has existed since KVM/ARM's inception. It's unlikely that any sane userspace currently modifies VMAs in such a way as to trigger this race. And even with directed testing I was unable to reproduce it. But a sufficiently motivated host userspace might be able to exploit this race.

Fixes: 94f8e6418d39 ("KVM: ARM: Handle guest faults in KVM") Cc: stable@vger.kernel.org Reported-by: Sean Christopherson seanjc@google.com Signed-off-by: David Matlack dmatlack@google.com --- arch/arm64/kvm/mmu.c | 48 +++++++++++++++++++------------------------- 1 file changed, 21 insertions(+), 27 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c index 7113587222ff..f54408355d1d 100644 --- a/arch/arm64/kvm/mmu.c +++ b/arch/arm64/kvm/mmu.c @@ -1217,6 +1217,20 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, return -EFAULT; }

+ /* + * Permission faults just need to update the existing leaf entry, + * and so normally don't require allocations from the memcache. The + * only exception to this is when dirty logging is enabled at runtime + * and a write fault needs to collapse a block entry into a table. + */ + if (fault_status != ESR_ELx_FSC_PERM || + (logging_active && write_fault)) { + ret = kvm_mmu_topup_memory_cache(memcache, + kvm_mmu_cache_min_pages(kvm)); + if (ret) + return ret; + } + /* * Let's check if we will get back a huge page backed by hugetlbfs, or * get block mapping for device MMIO region. @@ -1269,37 +1283,17 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, fault_ipa &= ~(vma_pagesize - 1);

gfn = fault_ipa >> PAGE_SHIFT; - mmap_read_unlock(current->mm); - - /* - * Permission faults just need to update the existing leaf entry, - * and so normally don't require allocations from the memcache. The - * only exception to this is when dirty logging is enabled at runtime - * and a write fault needs to collapse a block entry into a table. - */ - if (fault_status != ESR_ELx_FSC_PERM || - (logging_active && write_fault)) { - ret = kvm_mmu_topup_memory_cache(memcache, - kvm_mmu_cache_min_pages(kvm)); - if (ret) - return ret; - }

- mmu_seq = vcpu->kvm->mmu_invalidate_seq; /* - * Ensure the read of mmu_invalidate_seq happens before we call - * gfn_to_pfn_prot (which calls get_user_pages), so that we don't risk - * the page we just got a reference to gets unmapped before we have a - * chance to grab the mmu_lock, which ensure that if the page gets - * unmapped afterwards, the call to kvm_unmap_gfn will take it away - * from us again properly. This smp_rmb() interacts with the smp_wmb() - * in kvm_mmu_notifier_invalidate_<page|range_end>. + * Read mmu_invalidate_seq so that KVM can detect if the results of + * vma_lookup() or __gfn_to_pfn_memslot() become stale prior to + * acquiring kvm->mmu_lock. * - * Besides, __gfn_to_pfn_memslot() instead of gfn_to_pfn_prot() is - * used to avoid unnecessary overhead introduced to locate the memory - * slot because it's always fixed even @gfn is adjusted for huge pages. + * Rely on mmap_read_unlock() for an implicit smp_rmb(), which pairs + * with the smp_wmb() in kvm_mmu_invalidate_end(). */ - smp_rmb(); + mmu_seq = vcpu->kvm->mmu_invalidate_seq; + mmap_read_unlock(current->mm);

pfn = __gfn_to_pfn_memslot(memslot, gfn, false, false, NULL, write_fault, &writable, NULL);

base-commit: 96a4627dbbd48144a65af936b321701c70876026