On Fri, Oct 14, 2022 at 08:19:42PM +0200, Jann Horn wrote: > 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 I think here we will serialize against anon_vma/i_mmap lock in __do_munmap() -> unmap_region() -> free_pgtables() that A also holds. So I believe munmap() is safe, but MADV_DONTNEED (and its flavours) is not. > [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. -- Kiryl Shutsemau / Kirill A. Shutemov
