Re: [PATCH] mm: use correct VMA flags when freeing page-tables

[Andrew Morton]

On Thu, 21 Oct 2021 05:23:22 -0700 Nadav Amit <nadav.amit@xxxxxxxxx> wrote:

> From: Nadav Amit <namit@xxxxxxxxxx>
> 
> Consistent use of the mmu_gather interface requires a call to
> tlb_start_vma() and tlb_end_vma() for each VMA. free_pgtables() does not
> follow this pattern.
> 
> Certain architectures need tlb_start_vma() to be called in order for
> tlb_update_vma_flags() to update the VMA flags (tlb->vma_exec and
> tlb->vma_huge), which are later used for the proper TLB flush to be
> issued. Since tlb_start_vma() is not called, this can lead to the wrong
> VMA flags being used when the flush is performed.
> 
> Specifically, the munmap syscall would call unmap_region(), which unmaps
> the VMAs and then frees the page-tables. A flush is needed after
> the page-tables are removed to prevent page-walk caches from holding
> stale entries, but this flush would use the flags of the VMA flags of
> the last VMA that was flushed. This does not appear to be right.

Any thoughts on what the worst-case end-user cisible effects of this
would be?

Again, I'm wondering about the desirability of a -stable backport.

> Use tlb_start_vma() and tlb_end_vma() to prevent this from happening.
> This might lead to unnecessary calls to flush_cache_range() on certain
> arch's. If needed, a new flag can be added to mmu_gather to indicate
> that the flush is not needed.