Re: [PATCH 1/2] mm, compaction: make capture control handling safe wrt interrupts

[Hugh Dickins <hughd@google.com>]

On Tue, 16 Jun 2020, Vlastimil Babka wrote:

> Hugh reports:
> 
> =====
> While stressing compaction, one run oopsed on NULL capc->cc in
> __free_one_page()'s task_capc(zone): compact_zone_order() had been
> interrupted, and a page was being freed in the return from interrupt.
> 
> Though you would not expect it from the source, both gccs I was using
> (a 4.8.1 and a 7.5.0) had chosen to compile compact_zone_order() with
> the ".cc = &cc" implemented by mov %rbx,-0xb0(%rbp) immediately before
> callq compact_zone - long after the "current->capture_control = &capc".
> An interrupt in between those finds capc->cc NULL (zeroed by an earlier
> rep stos).
> 
> This could presumably be fixed by a barrier() before setting
> current->capture_control in compact_zone_order(); but would also need
> more care on return from compact_zone(), in order not to risk leaking
> a page captured by interrupt just before capture_control is reset.
> 
> Maybe that is the preferable fix, but I felt safer for task_capc() to
> exclude the rather surprising possibility of capture at interrupt time.
> =====
> 
> I have checked that gcc10 also behaves the same.
> 
> The advantage of fix in compact_zone_order() is that we don't add another
> test in the page freeing hot path, and that it might prevent future problems
> if we stop exposing pointers to unitialized structures in current task.
> 
> So this patch implements the suggestion for compact_zone_order() with barrier()
> (and WRITE_ONCE() to prevent store tearing) for setting
> current->capture_control, and prevents page leaking with WRITE_ONCE/READ_ONCE
> in the proper order.
> 
> Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
> Cc: stable@vger.kernel.org # 5.1+
> Reported-by: Hugh Dickins <hughd@google.com>
> Suggested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Hugh Dickins <hughd@google.com>

> ---
>  mm/compaction.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index fd988b7e5f2b..86375605faa9 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2316,15 +2316,26 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
>  		.page = NULL,
>  	};
>  
> -	current->capture_control = &capc;
> +	/*
> +	 * Make sure the structs are really initialized before we expose the
> +	 * capture control, in case we are interrupted and the interrupt handler
> +	 * frees a page.
> +	 */
> +	barrier();
> +	WRITE_ONCE(current->capture_control, &capc);
>  
>  	ret = compact_zone(&cc, &capc);
>  
>  	VM_BUG_ON(!list_empty(&cc.freepages));
>  	VM_BUG_ON(!list_empty(&cc.migratepages));
>  
> -	*capture = capc.page;
> -	current->capture_control = NULL;
> +	/*
> +	 * Make sure we hide capture control first before we read the captured
> +	 * page pointer, otherwise an interrupt could free and capture a page
> +	 * and we would leak it.
> +	 */
> +	WRITE_ONCE(current->capture_control, NULL);
> +	*capture = READ_ONCE(capc.page);
>  
>  	return ret;
>  }
> -- 
> 2.27.0