Re: [PATCH v1 1/1] KVM: PPC: Book3S: Protect TCE IOMMU table walks with RCU

[IBM]

Hi Narayana,

Narayana Murty N <nnmlinux@xxxxxxxxxxxxx> writes:

> Commit cad32d9d42e8 ("KVM: PPC: Book3s: Retire H_PUT_TCE/etc real mode
> handlers") moved the TCE hcalls from real mode to preemptible virtual
> mode.
>
> The stt->iommu_tables list is RCU managed. Entries are added with
> list_add_rcu(), removed with list_del_rcu(), and released after an RCU
> grace period with call_rcu().
>
> However, kvmppc_h_put_tce(), kvmppc_h_put_tce_indirect(), and
> kvmppc_h_stuff_tce() traverse iommu_tables using
> list_for_each_entry_lockless() without an RCU read-side critical
> section.
>
> A concurrent IOMMU table detach can therefore remove an entry while a
> TCE hcall is traversing or using it, without the traversal preventing
> the entry from being reclaimed.
>
> Use list_for_each_entry_rcu() under rcu_read_lock() for these walks so
> that an entry removed concurrently is not reclaimed until the reader
> has completed.
>
> Fixes: cad32d9d42e8 ("KVM: PPC: Book3s: Retire H_PUT_TCE/etc real mode handlers")

I am not sure if this is the right fix. On doing git blame, I see the
following patch introduced this:

121f80ba ("KVM: PPC: VFIO: Add in-kernel acceleration for VFIO") - Alexey Kardashevskiy


> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Narayana Murty N <nnmlinux@xxxxxxxxxxxxx>
> ---
>  arch/powerpc/kvm/book3s_64_vio.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)


Sashiko pointed out few more bugs around this path:

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] kvm: ppc: UAF of spapr_tce_table due to missing rcu_read_lock()
- [High] kvmppc_clear_tce: pinned memory leak on partial IOMMU map failure

So, even though these looks to be pre-existing bugs - do you think we
can address them in a single patch series itself? This will reduce your
testing/validation as well?
Also since we are anyway fixing RCU specific bugs in patch-1, so I think
we can also fix rcu handling for stt as well?

Do you mind working on v2 and addressing Sashiko review comments please?

-ritesh