Re: [PATCH] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

[Oleg Nesterov <oleg@redhat.com>]

On 11/22, Andrea Parri wrote:
>
> Commit 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() +
> register() race") added the UPROBE_COPY_INSN flag, and corresponding
> smp_wmb() and smp_rmb() memory barriers, to ensure that handle_swbp()
> uses fully-initialized uprobes only.
> 
> However, the smp_rmb() is mis-placed: this barrier should be placed
> after handle_swbp() has tested for the flag, thus guaranteeing that
> (program-order) subsequent loads from the uprobe can see the initial
> stores performed by prepare_uprobe().
> 
> Move the smp_rmb() accordingly.  Also amend the comments associated
> to the two memory barriers to indicate their actual locations.
> 
> Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: stable@kernel.org
> Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")

Thanks,

Acked-by: Oleg Nesterov <oleg@redhat.com>



> ---
>  kernel/events/uprobes.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 96d4bee83489b..322e97bbb4370 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -829,7 +829,7 @@ static int prepare_uprobe(struct uprobe *uprobe, struct file *file,
>  	BUG_ON((uprobe->offset & ~PAGE_MASK) +
>  			UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
>  
> -	smp_wmb(); /* pairs with rmb() in find_active_uprobe() */
> +	smp_wmb(); /* pairs with the smp_rmb() in handle_swbp() */
>  	set_bit(UPROBE_COPY_INSN, &uprobe->flags);
>  
>   out:
> @@ -2178,10 +2178,18 @@ static void handle_swbp(struct pt_regs *regs)
>  	 * After we hit the bp, _unregister + _register can install the
>  	 * new and not-yet-analyzed uprobe at the same address, restart.
>  	 */
> -	smp_rmb(); /* pairs with wmb() in install_breakpoint() */
>  	if (unlikely(!test_bit(UPROBE_COPY_INSN, &uprobe->flags)))
>  		goto out;
>  
> +	/*
> +	 * Pairs with the smp_wmb() in prepare_uprobe().
> +	 *
> +	 * Guarantees that if we see the UPROBE_COPY_INSN bit set, then
> +	 * we must also see the stores to &uprobe->arch performed by the
> +	 * prepare_uprobe() call.
> +	 */
> +	smp_rmb();
> +
>  	/* Tracing handlers use ->utask to communicate with fetch methods */
>  	if (!get_utask())
>  		goto out;
> -- 
> 2.17.1
>