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 <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Arnaldo Carvalho de Melo <[email protected]>
> Cc: Alexander Shishkin <[email protected]>
> Cc: Jiri Olsa <[email protected]>
> Cc: Namhyung Kim <[email protected]>
> Cc: Oleg Nesterov <[email protected]>
> Cc: [email protected]
> Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() +
> register() race")
Thanks,
Acked-by: Oleg Nesterov <[email protected]>
> ---
> 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
>
