linux-kernel - Re: [PATCHv2 mm-stable] uprobes: Revert ref_ctr_offset in uprobe

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Message-ID: <aECseBOkQynCpnfK@krava>
Date: Wed, 4 Jun 2025 22:28:40 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Masami Hiramatsu <mhiramat@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii@...nel.org>, g@...va.smtp.subspace.kernel.org
Cc: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCHv2 mm-stable] uprobes: Revert ref_ctr_offset in
 uprobe_unregister error path

On Wed, May 14, 2025 at 12:18:09PM +0200, Jiri Olsa wrote:
> From: Jiri Olsa <olsajiri@...il.com>
> 
> There's error path that could lead to inactive uprobe:
> 
>   1) uprobe_register succeeds - updates instruction to int3 and
>      changes ref_ctr from 0 to 1
>   2) uprobe_unregister fails  - int3 stays in place, but ref_ctr
>      is changed to 0 (it's not restored to 1 in the fail path)
>      uprobe is leaked
>   3) another uprobe_register comes and re-uses the leaked uprobe
>      and succeds - but int3 is already in place, so ref_ctr update
>      is skipped and it stays 0 - uprobe CAN NOT be triggered now
>   4) uprobe_unregister fails because ref_ctr value is unexpected
> 
> Fixing this by reverting the updated ref_ctr value back to 1 in step 2),
> which is the case when uprobe_unregister fails (int3 stays in place),
> but we have already updated refctr.
> 
> The new scenario will go as follows:
> 
>   1) uprobe_register succeeds - updates instruction to int3 and
>      changes ref_ctr from 0 to 1
>   2) uprobe_unregister fails  - int3 stays in place and ref_ctr
>      is reverted to 1..  uprobe is leaked
>   3) another uprobe_register comes and re-uses the leaked uprobe
>      and succeds - but int3 is already in place, so ref_ctr update
>      is skipped and it stays 1 - uprobe CAN be triggered now
>   4) uprobe_unregister succeeds
> 
> Fixes: 1cc33161a83d ("uprobes: Support SDT markers having reference count (semaphore)")
> Acked-by: David Hildenbrand <david@...hat.com>
> Acked-by: Oleg Nesterov <oleg@...hat.com>
> Suggested-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>

hi,
I can't find this in any related tree, was this pulled in?

thanks,
jirka


> ---
> v2 changes:
> - adding proper Fixes tag and acks
> 
>  kernel/events/uprobes.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 4c965ba77f9f..84ee7b590861 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -581,8 +581,8 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
>  
>  out:
>  	/* Revert back reference counter if instruction update failed. */
> -	if (ret < 0 && is_register && ref_ctr_updated)
> -		update_ref_ctr(uprobe, mm, -1);
> +	if (ret < 0 && ref_ctr_updated)
> +		update_ref_ctr(uprobe, mm, is_register ? -1 : 1);
>  
>  	/* try collapse pmd for compound page */
>  	if (ret > 0)
> -- 
> 2.49.0
>