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@xxxxxxxxxxxxxxx
Reported-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
---
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
--
2.40.0.rc1.284.g88254d51c5-goog
