Re: [PATCH bpf v6 3/4] bpf: Fix potential UAF when reading bpf link info

[Pu Lehui]

On 2026/7/28 7:15, Andrii Nakryiko wrote:
> On Wed, Jul 22, 2026 at 12:19 AM Pu Lehui <pulehui@xxxxxxxxxxxxxxx> wrote:
> > From: Pu Lehui <pulehui@xxxxxxxxxx>
> >
> > In bpf_link_show_fdinfo and bpf_link_get_info_by_fd, link->prog is
> > accessed without holding any locks. If the prog is concurrently replaced
> > via bpf_link_update, the old prog can be freed, leading to a potential
> > UAF issue.
> >
> > Before dereferencing the prog, both normal RCU and RCU Tasks Trace read
>
> no, either one is enough (and that's why we wait for RCU and RCU Tasks
> Trace grace periods, to make sure that both kinds of accesses work).
> This paragraph is misleading, just drop it.

ok

> > locks would normally be required, as BPF_LINK_TYPE_ITER supports both
> > non-sleepable and sleepable progs. However, as commit 57b23c0f612d
> > ("bpf: Retire rcu_trace_implies_rcu_gp()") clarifies, an RCU Tasks Trace
> > grace period implies an RCU grace period, so holding only
> > rcu_read_lock() is already sufficient.
> >
> > Fixes: 0c991ebc8c69 ("bpf: Implement bpf_prog replacement for an active bpf_cgroup_link")
> > Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> > Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
> > Reviewed-by: Amery Hung <ameryhung@xxxxxxxxx>
> > Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
> > ---
> >   kernel/bpf/syscall.c | 21 +++++++++++++++++----
> >   1 file changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 6db306d23b47..cad986807d53 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -3471,9 +3471,10 @@ static const char *bpf_link_type_strs[] = {
> >   static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
> >   {
> >          const struct bpf_link *link = filp->private_data;
> > -       const struct bpf_prog *prog = link->prog;
> > +       const struct bpf_prog *prog;
> >          enum bpf_link_type type = link->type;
> >          char prog_tag[sizeof(prog->tag) * 2 + 1] = { };
> > +       u32 prog_id;
>
> can be read uninitialized (if link->prog is null), initialize to zero

will send new

> pw-bot: cr
>
>
> >          if (type < ARRAY_SIZE(bpf_link_type_strs) && bpf_link_type_strs[type]) {
> >                  if (link->type == BPF_LINK_TYPE_KPROBE_MULTI)
> > @@ -3490,13 +3491,20 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
> >          }
> >          seq_printf(m, "link_id:\t%u\n", link->id);
> >
> > +       rcu_read_lock();
> > +       prog = READ_ONCE(link->prog);
> >          if (prog) {
> >                  bin2hex(prog_tag, prog->tag, sizeof(prog->tag));
> > +               prog_id = prog->aux->id;
> > +       }
> > +       rcu_read_unlock();
> > +
> > +       if (prog) {
> >                  seq_printf(m,
> >                             "prog_tag:\t%s\n"
> >                             "prog_id:\t%u\n",
> >                             prog_tag,
> > -                          prog->aux->id);
> > +                          prog_id);
> >          }
> >          if (link->ops->show_fdinfo)
> >                  link->ops->show_fdinfo(link, m);
> > @@ -5535,6 +5543,7 @@ static int bpf_link_get_info_by_fd(struct file *file,
> >   {
> >          struct bpf_link_info __user *uinfo = u64_to_user_ptr(attr->info.info);
> >          struct bpf_link_info info;
> > +       const struct bpf_prog *prog;
> >          u32 info_len = attr->info.info_len;
> >          int err;
> >
> > @@ -5549,8 +5558,12 @@ static int bpf_link_get_info_by_fd(struct file *file,
> >
> >          info.type = link->type;
> >          info.id = link->id;
> > -       if (link->prog)
> > -               info.prog_id = link->prog->aux->id;
> > +
> > +       rcu_read_lock();
> > +       prog = READ_ONCE(link->prog);
> > +       if (prog)
> > +               info.prog_id = prog->aux->id;
> > +       rcu_read_unlock();
> >
> >          if (link->ops->fill_link_info) {
> >                  err = link->ops->fill_link_info(link, &info);
> > --
> > 2.34.1