On Fri, Apr 21, 2023 at 7:06 PM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> On Sat, Apr 22, 2023 at 01:49:08AM CEST, Alexei Starovoitov wrote:
> > On Sat, Apr 22, 2023 at 12:17:47AM +0200, Kumar Kartikeya Dwivedi wrote:
> > > On Sat, Apr 15, 2023 at 10:18:11PM CEST, Dave Marchevsky wrote:
> > > > Test refcounted local kptr functionality added in previous patches in
> > > > the series.
> > > >
> > > > Usecases which pass verification:
> > > >
> > > > * Add refcounted local kptr to both tree and list. Then, read and -
> > > > possibly, depending on test variant - delete from tree, then list.
> > > > * Also test doing read-and-maybe-delete in opposite order
> > > > * Stash a refcounted local kptr in a map_value, then add it to a
> > > > rbtree. Read from both, possibly deleting after tree read.
> > > > * Add refcounted local kptr to both tree and list. Then, try reading and
> > > > deleting twice from one of the collections.
> > > > * bpf_refcount_acquire of just-added non-owning ref should work, as
> > > > should bpf_refcount_acquire of owning ref just out of bpf_obj_new
> > > >
> > > > Usecases which fail verification:
> > > >
> > > > * The simple successful bpf_refcount_acquire cases from above should
> > > > both fail to verify if the newly-acquired owning ref is not dropped
> > > >
> > > > Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
> > > > ---
> > > > [...]
> > > > +SEC("?tc")
> > > > +__failure __msg("Unreleased reference id=3 alloc_insn=21")
> > > > +long rbtree_refcounted_node_ref_escapes(void *ctx)
> > > > +{
> > > > + struct node_acquire *n, *m;
> > > > +
> > > > + n = bpf_obj_new(typeof(*n));
> > > > + if (!n)
> > > > + return 1;
> > > > +
> > > > + bpf_spin_lock(&glock);
> > > > + bpf_rbtree_add(&groot, &n->node, less);
> > > > + /* m becomes an owning ref but is never drop'd or added to a tree */
> > > > + m = bpf_refcount_acquire(n);
> > >
> > > I am analyzing the set (and I'll reply in detail to the cover letter), but this
> > > stood out.
> > >
> > > Isn't this going to be problematic if n has refcount == 1 and is dropped
> > > internally by bpf_rbtree_add? Are we sure this can never occur? It took me some
> > > time, but the following schedule seems problematic.
> > >
> > > CPU 0 CPU 1
> > > n = bpf_obj_new
> > > lock(lock1)
> > > bpf_rbtree_add(rbtree1, n)
> > > m = bpf_rbtree_acquire(n)
> > > unlock(lock1)
> > >
> > > kptr_xchg(map, m) // move to map
> > > // at this point, refcount = 2
> > > m = kptr_xchg(map, NULL)
> > > lock(lock2)
> > > lock(lock1) bpf_rbtree_add(rbtree2, m)
> > > p = bpf_rbtree_first(rbtree1) if (!RB_EMPTY_NODE) bpf_obj_drop_impl(m) // A
> > > bpf_rbtree_remove(rbtree1, p)
> > > unlock(lock1)
> > > bpf_obj_drop(p) // B
> >
> > You probably meant:
> > p2 = bpf_rbtree_remove(rbtree1, p)
> > unlock(lock1)
> > if (p2)
> > bpf_obj_drop(p2)
> >
> > > bpf_refcount_acquire(m) // use-after-free
> > > ...
> > >
> > > B will decrement refcount from 1 to 0, after which bpf_refcount_acquire is
> > > basically performing a use-after-free (when fortunate, one will get a
> > > WARN_ON_ONCE splat for 0 to 1, otherwise, a silent refcount raise for some
> > > different object).
> >
> > As discussed earlier we'll be switching all bpf_obj_new to use BPF_MA_REUSE_AFTER_RCU_GP
>
> I probably missed that thread. In that case, is use of this stuff going to
> require bpf_rcu_read_lock in sleepable programs?
Correct.
We've refactored rcu enforcement.
We also decided to stop waiting for gcc btf_tag support to land and
went with allowlisting BTF_TYPE_SAFE_RCU[_OR_NULL] for fields that
we need to use now.
Overall it allowed us to get rid of kptr_get.
