Re: [PATCHv2 mm-stable] uprobes: Revert ref_ctr_offset in uprobe_unregister error path

[Jiri Olsa <olsajiri@gmail.com>]

hi, ping ;-)

On Wed, Jun 04, 2025 at 10:28:42PM +0200, Jiri Olsa wrote:
> On Wed, May 14, 2025 at 12:18:09PM +0200, Jiri Olsa wrote:
> > From: Jiri Olsa <olsajiri@gmail.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@redhat.com>
> > Acked-by: Oleg Nesterov <oleg@redhat.com>
> > Suggested-by: Oleg Nesterov <oleg@redhat.com>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.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
> >