selftests/bpf: Reject graph kptr use after RCU unlockgrafted

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 142 insertions, 0 deletions

diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
new file mode 100644
index 000000000..ae742564a
--- /dev/null
+++ b/arch/powerpc/mm/pgtable-frag.c
@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ *  Handling Page Tables through page fragments
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/gfp.h>
+#include <linux/mm.h>
+#include <linux/percpu.h>
+#include <linux/hardirq.h>
+#include <linux/hugetlb.h>
+#include <asm/pgalloc.h>
+#include <asm/tlbflush.h>
+#include <asm/tlb.h>
+
+void pte_frag_destroy(void *pte_frag)
+{
+	int count;
+	struct ptdesc *ptdesc;
+
+	ptdesc = virt_to_ptdesc(pte_frag);
+	/* drop all the pending references */
+	count = ((unsigned long)pte_frag & ~PAGE_MASK) >> PTE_FRAG_SIZE_SHIFT;
+	/* We allow PTE_FRAG_NR fragments from a PTE page */
+	if (atomic_sub_and_test(PTE_FRAG_NR - count, &ptdesc->pt_frag_refcount)) {
+		folio_clear_active(ptdesc_folio(ptdesc));
+		pagetable_dtor(ptdesc);
+		pagetable_free(ptdesc);
+	}
+}
+
+static pte_t *get_pte_from_cache(struct mm_struct *mm)
+{
+	void *pte_frag, *ret;
+
+	if (PTE_FRAG_NR == 1)
+		return NULL;
+
+	spin_lock(&mm->page_table_lock);
+	ret = pte_frag_get(&mm->context);
+	if (ret) {
+		pte_frag = ret + PTE_FRAG_SIZE;
+		/*
+		 * If we have taken up all the fragments mark PTE page NULL
+		 */
+		if (((unsigned long)pte_frag & ~PAGE_MASK) == 0)
+			pte_frag = NULL;
+		pte_frag_set(&mm->context, pte_frag);
+	}
+	spin_unlock(&mm->page_table_lock);
+	return (pte_t *)ret;
+}
+
+static pte_t *__alloc_for_ptecache(struct mm_struct *mm, int kernel)
+{
+	void *ret = NULL;
+	struct ptdesc *ptdesc;
+	gfp_t gfp = PGALLOC_GFP;
+
+	if (!kernel)
+		gfp |= __GFP_ACCOUNT;
+
+	ptdesc = pagetable_alloc(gfp, 0);
+	if (!ptdesc)
+		return NULL;
+	if (!pagetable_pte_ctor(mm, ptdesc)) {
+		pagetable_free(ptdesc);
+		return NULL;
+	}
+
+	atomic_set(&ptdesc->pt_frag_refcount, 1);
+
+	ret = ptdesc_address(ptdesc);
+	/*
+	 * if we support only one fragment just return the
+	 * allocated page.
+	 */
+	if (PTE_FRAG_NR == 1)
+		return ret;
+	spin_lock(&mm->page_table_lock);
+	/*
+	 * If we find ptdesc_page set, we return
+	 * the allocated page with single fragment
+	 * count.
+	 */
+	if (likely(!pte_frag_get(&mm->context))) {
+		atomic_set(&ptdesc->pt_frag_refcount, PTE_FRAG_NR);
+		pte_frag_set(&mm->context, ret + PTE_FRAG_SIZE);
+	}
+	spin_unlock(&mm->page_table_lock);
+
+	return (pte_t *)ret;
+}
+
+pte_t *pte_fragment_alloc(struct mm_struct *mm, int kernel)
+{
+	pte_t *pte;
+
+	pte = get_pte_from_cache(mm);
+	if (pte)
+		return pte;
+
+	return __alloc_for_ptecache(mm, kernel);
+}
+
+static void pte_free_now(struct rcu_head *head)
+{
+	struct ptdesc *ptdesc;
+
+	ptdesc = container_of(head, struct ptdesc, pt_rcu_head);
+	pagetable_dtor(ptdesc);
+	pagetable_free(ptdesc);
+}
+
+void pte_fragment_free(unsigned long *table, int kernel)
+{
+	struct ptdesc *ptdesc = virt_to_ptdesc(table);
+
+	if (pagetable_is_reserved(ptdesc))
+		return free_reserved_ptdesc(ptdesc);
+
+	BUG_ON(atomic_read(&ptdesc->pt_frag_refcount) <= 0);
+	if (atomic_dec_and_test(&ptdesc->pt_frag_refcount)) {
+		if (kernel || !folio_test_clear_active(ptdesc_folio(ptdesc)))
+			pte_free_now(&ptdesc->pt_rcu_head);
+		else
+			call_rcu(&ptdesc->pt_rcu_head, pte_free_now);
+	}
+}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
+{
+	struct folio *folio;
+
+	folio = virt_to_folio(pgtable);
+	folio_set_active(folio);
+	pte_fragment_free((unsigned long *)pgtable, 0);
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */