Re: [PATCH bpf-next v5 2/2] bpf: fix stackmap overflow check in __bpf_get_stackid()

[Lecomte, Arnaud]

On 30/08/2025 02:28, Alexei Starovoitov wrote:
> On Fri, Aug 29, 2025 at 11:50 AM Song Liu <song@xxxxxxxxxx> wrote:
> > On Fri, Aug 29, 2025 at 10:29 AM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > [...]
> > > >   static long __bpf_get_stackid(struct bpf_map *map,
> > > > -                             struct perf_callchain_entry *trace, u64 flags)
> > > > +                             struct perf_callchain_entry *trace, u64 flags, u32 max_depth)
> > > >   {
> > > >          struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
> > > >          struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
> > > > @@ -263,6 +263,8 @@ static long __bpf_get_stackid(struct bpf_map *map,
> > > >
> > > >          trace_nr = trace->nr - skip;
> > > >          trace_len = trace_nr * sizeof(u64);
> > > > +       trace_nr = min(trace_nr, max_depth - skip);
> > > > +
> > > The patch might have fixed this particular syzbot repro
> > > with OOB in stackmap-with-buildid case,
> > > but above two line looks wrong.
> > > trace_len is computed before being capped by max_depth.
> > > So non-buildid case below is using
> > > memcpy(new_bucket->data, ips, trace_len);
> > >
> > > so OOB is still there?
> > +1 for this observation.
> >
> > We are calling __bpf_get_stackid() from two functions: bpf_get_stackid
> > and bpf_get_stackid_pe. The check against max_depth is only needed
> > from bpf_get_stackid_pe, so it is better to just check here.
> Good point.
Nice catch, thanks !
> > I have got the following on top of patch 1/2. This makes more sense to
> > me.
> >
> > PS: The following also includes some clean up in __bpf_get_stack.
> > I include those because it also uses stack_map_calculate_max_depth.
> >
> > Does this look better?
> yeah. It's certainly cleaner to avoid adding extra arg to
> __bpf_get_stackid()
Are Song patches going to be applied then ?  Or should I raise a new 
revision
 of the patch with Song's modifications with a Co-developped tag ?
Thanks for your guidance in advance,
Arnaud