Re: [PATCH mm-stable] uprobes: Revert ref_ctr_offset in uprobe_unregister error path

[David Hildenbrand <david@redhat.com>]

On 13.05.25 14:21, Jiri Olsa wrote:
> From: Jiri Olsa <olsajiri@gmail.com>
> 

Thanks for debugging.

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

If it's in mm-stable, we should have

Fixes: ...

here

> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> Please note it's based on mm-stable branch, because it has the
> latest uprobe_write_opcode rewrite changes.
> 
>   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);


Hm, but my patch essentially did here

         /* Revert back reference counter if instruction update failed. */
-       if (ret && is_register && ref_ctr_updated)
+       if (ret < 0 && is_register && ref_ctr_updated)
                 update_ref_ctr(uprobe, mm, -1);

So how come this wasn't a problem before?

-- 
Cheers,

David / dhildenb