Re: [PATCH v2] KVM: arm/arm64: Handle hva aging while destroying the vm

From: Suzuki K Poulose
Date: Thu Jul 06 2017 - 05:35:17 EST

On 06/07/17 08:45, Christoffer Dall wrote:
On Thu, Jul 06, 2017 at 09:07:49AM +0200, Alexander Graf wrote:

On 05.07.17 10:57, Suzuki K Poulose wrote:
Hi Alex,

On Wed, Jul 05, 2017 at 08:20:31AM +0200, Alexander Graf wrote:
The kvm_age_hva callback may be called all the way concurrently while
kvm_mmu_notifier_release() is running.

The release function sets kvm->arch.pgd = NULL which the aging function
however implicitly relies on in stage2_get_pud(). That means they can
race and the aging function may dereference a NULL pgd pointer.

This patch adds a check for that case, so that we leave the aging
function silently.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 293f29363 ("kvm-arm: Unmap shadow pagetables properly")
Signed-off-by: Alexander Graf <agraf@xxxxxxx>


v1 -> v2:

- Fix commit message
- Add Fixes and stable tags
virt/kvm/arm/mmu.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index f2d5b6c..227931f 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -861,6 +861,10 @@ static pud_t *stage2_get_pud(struct kvm *kvm, struct kvm_mmu_memory_cache *cache
pgd_t *pgd;
pud_t *pud;
+ /* Do we clash with kvm_free_stage2_pgd()? */
+ if (!kvm->arch.pgd)
+ return NULL;

I think this check should be moved up in the chain. We call kvm_age_hva(), with
the kvm->mmu_lock held and we don't release it till we reach here. So, ideally,
if we find the PGD is null when we reach kvm_age_hva(), we could simply return
there, like we do for other call backs from the KVM mmu_notifier.

That probably works too - I'm not sure which version is more
consistent as well as more maintainable in the long run. I'll leave
the call here to Christoffer.

Let's look at the callers to stage2_get_pmd, which is the only caller of
stage2_get_pud, where the problem was observed:

-> stage2_set_pmd_huge
-> stage2_get_pmd

-> stage2_set_pte
-> stage2_get_pmd

-> stage2_get_pmd

For the above three functions, pgd cannot ever be NULL, because this is
running in the context of a VCPU thread, which means the reference on
the VM fd must not reach zero, so no need to call that here.

I think there is some problem here. See below for more information.

-> stage2_set_pte
-> stage2_get_pmd

This is called from kvm_set_spte_hva, which is one of the MMU notifiers,
so it can race similarly kvm_age_hva and kvm_test_age_hva, but it
already checks for !kvm->arch.pgd.

-> stage2_set_pte
-> stage2_get_pmd

This is called from two places: (1) The VGIC code (as part of
vgic_v2_map_resources) and can only be called in the context of running
a VCPU, so the pgd cannot be null by virtue of the same argument as for
user_mem_abort. (2) kvm_arch_prepare_memory_region calls
kvm_phys_addr_ioremap, which is a VM ioctl so similarly, I cannot see
how the VM can be in the middle of being freed while handling ioctls on
the fd. Therefore, following the same argument, this should be safe as

kvm_age_hva_handler and kvm_test_age_hva_handler
-> stage2_get_pmd

Handled by the patch proposed by Suzuki.

What does all that tell us? First, it should give us some comfort that we
don't have more races of this kind. Second, it teels us that there are
a number of different and not-obvious call paths to stage2_pet_pud,
which could be an argument to simply check the pgd whenever it's called,
despite the minor runtime overhead. On the other hand, the check itself
is only valid knowing that we synchronize against kvm_free_stage2_pgd
using the kvm->mmu_lock() and understanding that this only happens when
mmu notifiers call into the KVM MMU code outside the context of the VM.

The last consideration is the winning argument for me to put the check
in kvm_age_hva_handler and kvm_test_age_hva_handler, but I think it's
important that we document why it's only these three high-level callers
(incl. kvm_set_spte_handler) that need the check, either in the code or
in the commit message.

The only way we end up freeing the stage-2 PGD is via the mmu_notifier_release(),
which could be triggered via two different paths.

1) kvm_destroy_vm(), where all the VM resources has been released and the
refcount on the KVM instances are dropped, via kvm_put_kvm().

kvm_free_stage2_pgd -> free the page table with the mmu_lock held
occasionally releasing it to avoid contention.

2) do_signal -> get_signal -> do_group_exit - >
mmput => __mmput

In the first case, all references to the VM are dropped and hence none of the
VCPU could still be executing.

However, in the second case it may not be. So we have a potential problem with
the VCPU trying to run even when the pages were unmapped. I think the root cause
of all these issues boils down to the assumption that KVM holds a reference to
MM (which is not necessarily the user space mapping. i.e mmgrab vs mmget).
I am not sure if the VCPU should hold a reference to the mmaps to make sure
it is safe to run. That way, the mmap stays until the VCPU eventually exits
and we are safe all the way around.