Re: [PATCH bpf] bpf: Reject writes through untrusted allocated pointers

[Ning Ding]

The existing PTR_UNTRUSTED branch is after the access-permission
check. It only handles the case where we propagate the PTR_UNTRUSTED
flag to a field pointer loaded through a BPF_READ. For a direct write
such as res->key = 42, no pointer is loaded and that branch has no
effect:

my_child = res->child; // in that case, my_child will be marked with
PTR_UNTRUSTED, that is what the existing PTR_UNTRUSTED do.
res->key = 42; // The existing check doesn't handle this.

On Sun, Aug 2, 2026 at 4:32 PM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
>
> On Tue Jul 28, 2026 at 5:38 AM CEST, Ning Ding wrote:
> > A kptr loaded under an RCU read lock has MEM_ALLOC set. After
> > bpf_rcu_read_unlock(), the verifier marks it PTR_UNTRUSTED, but
> > check_ptr_to_btf_access() still allows writes because
> > type_is_ptr_alloc_obj() ignores that flag.
> >
> > Reject writes through MEM_ALLOC pointers that are PTR_UNTRUSTED. Reads
> > remain allowed and are converted to probe-memory accesses.
> >
> > Also skip the owning-reference check for these pointers, since they no
> > longer have a live reference after leaving the RCU critical section.
> >
> > Add verifier tests for the rejected write and permitted read cases.
> >
> > Fixes: 503e4def5414 ("bpf: Replace open code with for allocated object check")
> > Closes: https://lore.kernel.org/all/20260726021304.97ED91F000E9@xxxxxxxxxxxxxxx/
> > Assisted-by: Codex:gpt-5.6-sol
> > Signed-off-by: Ning Ding <dingning04@xxxxxxxxx>
> > ---
>
> Same comment for this one, split kernel and selftests into separate patches.
>
> >  kernel/bpf/verifier.c                         |  6 +-
> >  .../bpf/progs/local_kptr_stash_fail.c         | 58 +++++++++++++++++++
> >  2 files changed, 62 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 7aa47342dc65..24c151ad2e62 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5804,13 +5804,15 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
> >                * program allocated objects (which always have id > 0),
> >                * but not for untrusted PTR_TO_BTF_ID | MEM_ALLOC.
> >                */
> > -             if (atype != BPF_READ && !type_is_ptr_alloc_obj(reg->type)) {
> > +             if (atype != BPF_READ &&
> > +                 (!type_is_ptr_alloc_obj(reg->type) || reg->type & PTR_UNTRUSTED)) {
> >                       verbose(env, "only read is supported\n");
> >                       return -EACCES;
> >               }
>
> There is a PTR_UNTRUSTED check already earlier in this function, why is that not
> working or not enough?
>
> pw-bot: cr
>
> >
> >               if (type_is_alloc(reg->type) && !type_is_non_owning_ref(reg->type) &&
> > -                 !(reg->type & MEM_RCU) && !reg_is_referenced(env, reg)) {
> > +                 !(reg->type & (MEM_RCU | PTR_UNTRUSTED)) &&
> > +                 !reg_is_referenced(env, reg)) {
> >                       verifier_bug(env, "allocated object must have a referenced id");
> >                       return -EFAULT;
> >               }
> > diff --git a/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c b/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c
> > index fcf7a7567da2..d7b8cd6458bb 100644
> > --- a/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c
> > +++ b/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c
> > @@ -8,14 +8,22 @@
> >  #include "../bpf_experimental.h"
> >  #include "bpf_misc.h"
> >
> > +extern void bpf_rcu_read_lock(void) __ksym;
> > +extern void bpf_rcu_read_unlock(void) __ksym;
> > +
> >  struct node_data {
> >       long key;
> >       long data;
> >       struct bpf_rb_node node;
> >  };
> >
> > +struct plain_data {
> > +     long key;
> > +};
> > +
> >  struct map_value {
> >       struct node_data __kptr *node;
> > +     struct plain_data __kptr *plain;
> >  };
> >
> >  struct node_data2 {
> > @@ -31,6 +39,7 @@ struct node_data2 {
> >   * [35] TYPE_TAG 'kptr_ref' type_id=34
> >   */
> >  struct node_data *just_here_because_btf_bug;
> > +struct plain_data *just_here_because_btf_bug2;
> >
> >  struct {
> >       __uint(type, BPF_MAP_TYPE_ARRAY);
> > @@ -82,4 +91,53 @@ long drop_rb_node_off(void *ctx)
> >       return 0;
> >  }
> >
> > +SEC("?tc")
> > +__failure __msg("only read is supported")
> > +long write_untrusted_alloc_obj(void *ctx)
> > +{
> > +     struct map_value *mapval;
> > +     struct node_data *res;
> > +     int idx = 0;
> > +
> > +     mapval = bpf_map_lookup_elem(&some_nodes, &idx);
> > +     if (!mapval)
> > +             return 1;
> > +
> > +     bpf_rcu_read_lock();
> > +     res = mapval->node;
> > +     if (!res) {
> > +             bpf_rcu_read_unlock();
> > +             return 2;
> > +     }
> > +     barrier_var(res);
> > +     bpf_rcu_read_unlock();
> > +
> > +     res->key = 42;
> > +     return 0;
> > +}
> > +
> > +SEC("?tc")
> > +__success
> > +long read_untrusted_alloc_obj(void *ctx)
> > +{
> > +     struct map_value *mapval;
> > +     struct plain_data *res;
> > +     int idx = 0;
> > +
> > +     mapval = bpf_map_lookup_elem(&some_nodes, &idx);
> > +     if (!mapval)
> > +             return 1;
> > +
> > +     bpf_rcu_read_lock();
> > +     res = mapval->plain;
> > +     if (!res) {
> > +             bpf_rcu_read_unlock();
> > +             return 2;
> > +     }
> > +     barrier_var(res);
> > +     bpf_rcu_read_unlock();
> > +
> > +     return res->key;
> > +}
> > +
> >  char _license[] SEC("license") = "GPL";
>