[BUG?] X86 arch_tlbbatch_flush() seems to be lacking mm_tlb_flush_nested() integration

[Jann Horn]

Hi!

I haven't actually managed to reproduce this behavior, so maybe I'm
just misunderstanding how this works; but I think the
arch_tlbbatch_flush() path for batched TLB flushing in vmscan ought to
have some kind of integration with mm_tlb_flush_nested().

I think that currently, the following race could happen:

[initial situation: page P is mapped into a page table of task B, but
the page is not referenced, the PTE's A/D bits are clear]
A: vmscan begins
A: vmscan looks at P and P's PTEs, and concludes that P is not currently in use
B: reads from P through the PTE, setting the Accessed bit and creating
a TLB entry
A: vmscan enters try_to_unmap_one()
A: try_to_unmap_one() calls should_defer_flush(), which returns true
A: try_to_unmap_one() removes the PTE and queues a TLB flush
(arch_tlbbatch_add_mm())
A: try_to_unmap_one() returns, try_to_unmap() returns to shrink_folio_list()
B: calls munmap() on the VMA that mapped P
B: no PTEs are removed, so no TLB flush happens
B: munmap() returns
[at this point, the TLB entry still exists]
B: calls mmap(), which reuses the same area that was just unmapped
B: tries to access the newly created VMA, but instead the access goes
through the stale TLB entry
A: shrink_folio_list() calls try_to_unmap_flush(), which removes the
stale TLB entry

The effect would be that after process B removes a mapping with
munmap() and creates a new mapping in its place, it would still see
data from the old mapping when trying to access the new mapping.

Am I missing something that protects against this scenario?

munmap() uses the mmu_gather infrastructure, which tries to protect
against this kind of correctness bug with multiple racing TLB
invalidations in tlb_finish_mmu() by blowing away the whole TLB
whenever one TLB invalidation ends while another is still in progress
(tested with mm_tlb_flush_nested(tlb->mm)). But mmu_gather doesn't
seem to be aware of TLB flushes that are batched up in the
arch_tlbbatch_flush() infrastructure, so that doesn't help here.

I think it might be necessary to add a new global counter of pending
arch_tlbbatch_flush() flushes, and query that in
mm_tlb_flush_nested(), or something like that.