Re: [PATCH] kprobes: Fix to handle forcibly unoptimized kprobes on freeing_list

[Steven Rostedt]

On Mon, 23 Jan 2023 22:24:05 +0900
"Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> Sinec forcibly unoptimized kprobes will be put on the freeing_list directly

"Since"

> in the unoptimize_kprobe(), do_unoptimize_kprobes() must continue to check
> the freeing_list even if unoptimizing_list is empty.
> 
> This bug can be happen if a kprobe is put in an instruction which is in the

"This bug can happen if"

> middle of the jump-replaced instruction sequence of an optprobe, *and* the
> optprobe is recently unregistered and queued on unoptimizing_list.
> In this case, the optprobe will be unoptimized forcibly (means immediately)
> and put it into the freeing_list, expecting the optprobe will be handled in
> do_unoptimize_kprobe().
> But if there is no other optprobes on the unoptimizing_list, current code
> returns from the do_unoptimize_kprobe() soon and do not handle the optprobe

                                              "and does not handle'

> which is on the freeing_list, and it will hit the WARN_ON_ONCE() in the
> do_free_cleaned_kprobes(), because it is not handled in the latter loop of
> the do_unoptimize_kprobe().
> 
> To solve this issue, do not return from do_unoptimize_kprobes() immediately
> even if unoptimizing_list is empty.
> 
> Moreover, this change affects another case. kill_optimized_kprobes() expects
> kprobe_optimizer() will just free the optprobe on freeing_list.
> So I changed it to just do list_move() to freeing_list if optprobes are on
> unoptimizing list. And the do_unoptimize_kprobe() will skip
> arch_disarm_kprobe() if the probe on freeing_list has gone flag.
> 
> Link: https://lore.kernel.org/all/Y8URdIfVr3pq2X8w@xxxxxxxxxxxxxxxx/
> 
> Fixes: e4add247789e ("kprobes: Fix optimize_kprobe()/unoptimize_kprobe() cancellation logic")
> Reported-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
>  kernel/kprobes.c |   23 ++++++++++-------------
>  1 file changed, 10 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 1c18ecf9f98b..73b150fad936 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -555,17 +555,15 @@ static void do_unoptimize_kprobes(void)
>  	/* See comment in do_optimize_kprobes() */
>  	lockdep_assert_cpus_held();
>  
> -	/* Unoptimization must be done anytime */
> -	if (list_empty(&unoptimizing_list))
> -		return;
> +	if (!list_empty(&unoptimizing_list))
> +		arch_unoptimize_kprobes(&unoptimizing_list, &freeing_list);
>  
> -	arch_unoptimize_kprobes(&unoptimizing_list, &freeing_list);
> -	/* Loop on 'freeing_list' for disarming */
> +	/* Loop on 'freeing_list' for disarming and removing from kprobe hash list */
>  	list_for_each_entry_safe(op, tmp, &freeing_list, list) {
>  		/* Switching from detour code to origin */
>  		op->kp.flags &= ~KPROBE_FLAG_OPTIMIZED;
> -		/* Disarm probes if marked disabled */
> -		if (kprobe_disabled(&op->kp))
> +		/* Disarm probes if marked disabled and not gone */
> +		if (kprobe_disabled(&op->kp) && !kprobe_gone(&op->kp))
>  			arch_disarm_kprobe(&op->kp);
>  		if (kprobe_unused(&op->kp)) {
>  			/*
> @@ -797,14 +795,13 @@ static void kill_optimized_kprobe(struct kprobe *p)
>  	op->kp.flags &= ~KPROBE_FLAG_OPTIMIZED;
>  
>  	if (kprobe_unused(p)) {
> -		/* Enqueue if it is unused */
> -		list_add(&op->list, &freeing_list);
>  		/*
> -		 * Remove unused probes from the hash list. After waiting
> -		 * for synchronization, this probe is reclaimed.
> -		 * (reclaiming is done by do_free_cleaned_kprobes().)
> +		 * Unused kprobe is on unoptimizing or freeing list. We move it
> +		 * to freeing_list and let the kprobe_optimizer() removes it from

                                                                 "remove it"

> +		 * the kprobe hash list and frees it.

                                        "and free it."

>  		 */
> -		hlist_del_rcu(&op->kp.hlist);
> +		if (optprobe_queued_unopt(op))
> +			list_move(&op->list, &freeing_list);
>  	}
>  
>  	/* Don't touch the code, because it is already freed. */

Other than the spelling issues,

Acked-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

-- Steve