Re: [PATCH v2 bpf-next 9/9] selftests/bpf: Add refcounted_kptr tests

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

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@fb.com>
> ---
> [...]
> +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
					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).