Re: [PATCH 3/4] mmu_notifiers: Call arch_invalidate_secondary_tlbs() when invalidating TLBs

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 18 Jul 2023 17:56:17 +1000 Alistair Popple <apopple@nvidia.com> wrote:

> The arch_invalidate_secondary_tlbs() is an architecture specific mmu
> notifier used to keep the TLB of secondary MMUs such as an IOMMU in
> sync with the CPU page tables. Currently it is called from separate
> code paths to the main CPU TLB invalidations. This can lead to a
> secondary TLB not getting invalidated when required and makes it hard
> to reason about when exactly the secondary TLB is invalidated.
> 
> To fix this move the notifier call to the architecture specific TLB
> maintenance functions for architectures that have secondary MMUs
> requiring explicit software invalidations.
> 
> This fixes a SMMU bug on ARM64. On ARM64 PTE permission upgrades
> require a TLB invalidation. This invalidation is done by the
> architecutre specific ptep_set_access_flags() which calls
> flush_tlb_page() if required. However this doesn't call the notifier
> resulting in infinite faults being generated by devices using the SMMU
> if it has previously cached a read-only PTE in it's TLB.

This sounds like a pretty serious bug.  Can it happen in current
released kernels?  If so, is a -stable backport needed?

> Moving the invalidations into the TLB invalidation functions ensures
> all invalidations happen at the same time as the CPU invalidation. The
> architecture specific flush_tlb_all() routines do not call the
> notifier as none of the IOMMUs require this.