KVM: RISC-V: Retry fault if vma_lookup() results become invalid

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).

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.

Note KVM/ARM had the same bug and was fixed in a separate, near
identical patch (see Link).

Link: https://lore.kernel.org/kvm/20230313235454.2964067-1-dmatlack@google.com/
Fixes: 9955371cc014 ("RISC-V: KVM: Implement MMU notifiers")
Cc: stable@vger.kernel.org
Signed-off-by: David Matlack <dmatlack@google.com>
Tested-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>

diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
index 78211aed36fa6a3dcb2785e68442272131ded9c5..46d6929958306002accd5ad5586989915c4dbe0a 100644 (file)
--- a/arch/riscv/kvm/mmu.c
+++ b/arch/riscv/kvm/mmu.c
@@ -628,6 +628,13 @@ int kvm_riscv_gstage_map(struct kvm_vcpu *vcpu,
                        !(memslot->flags & KVM_MEM_READONLY)) ? true : false;
        unsigned long vma_pagesize, mmu_seq;
 
+       /* We need minimum second+third level pages */
+       ret = kvm_mmu_topup_memory_cache(pcache, gstage_pgd_levels);
+       if (ret) {
+               kvm_err("Failed to topup G-stage cache\n");
+               return ret;
+       }
+
        mmap_read_lock(current->mm);
 
        vma = vma_lookup(current->mm, hva);
@@ -648,6 +655,15 @@ int kvm_riscv_gstage_map(struct kvm_vcpu *vcpu,
        if (vma_pagesize == PMD_SIZE || vma_pagesize == PUD_SIZE)
                gfn = (gpa & huge_page_mask(hstate_vma(vma))) >> PAGE_SHIFT;
 
+       /*
+        * Read mmu_invalidate_seq so that KVM can detect if the results of
+        * vma_lookup() or gfn_to_pfn_prot() become stale priort to acquiring
+        * kvm->mmu_lock.
+        *
+        * Rely on mmap_read_unlock() for an implicit smp_rmb(), which pairs
+        * with the smp_wmb() in kvm_mmu_invalidate_end().
+        */
+       mmu_seq = kvm->mmu_invalidate_seq;
        mmap_read_unlock(current->mm);
 
        if (vma_pagesize != PUD_SIZE &&
@@ -657,15 +673,6 @@ int kvm_riscv_gstage_map(struct kvm_vcpu *vcpu,
                return -EFAULT;
        }
 
-       /* We need minimum second+third level pages */
-       ret = kvm_mmu_topup_memory_cache(pcache, gstage_pgd_levels);
-       if (ret) {
-               kvm_err("Failed to topup G-stage cache\n");
-               return ret;
-       }
-
-       mmu_seq = kvm->mmu_invalidate_seq;
-
        hfn = gfn_to_pfn_prot(kvm, gfn, is_write, &writable);
        if (hfn == KVM_PFN_ERR_HWPOISON) {
                send_sig_mceerr(BUS_MCEERR_AR, (void __user *)hva,