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

[Jiri Olsa <olsajiri@gmail.com>]

ping

On Wed, Jun 11, 2025 at 10:57:08AM +0200, Jiri Olsa wrote:
> 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
> > >