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, 73 insertions, 0 deletions

diff --git a/include/asm-generic/pgtable-nopmd.h b/include/asm-generic/pgtable-nopmd.h
new file mode 100644
index 000000000..8ffd64e7a
--- /dev/null
+++ b/include/asm-generic/pgtable-nopmd.h
@@ -0,0 +1,73 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _PGTABLE_NOPMD_H
+#define _PGTABLE_NOPMD_H
+
+#ifndef __ASSEMBLY__
+
+#include <asm-generic/pgtable-nopud.h>
+
+struct mm_struct;
+
+#define __PAGETABLE_PMD_FOLDED 1
+
+/*
+ * Having the pmd type consist of a pud gets the size right, and allows
+ * us to conceptually access the pud entry that this pmd is folded into
+ * without casting.
+ */
+typedef struct { pud_t pud; } pmd_t;
+
+#define PMD_SHIFT	PUD_SHIFT
+#define PTRS_PER_PMD	1
+#define PMD_SIZE  	(1UL << PMD_SHIFT)
+#define PMD_MASK  	(~(PMD_SIZE-1))
+
+/*
+ * The "pud_xxx()" functions here are trivial for a folded two-level
+ * setup: the pmd is never bad, and a pmd always exists (as it's folded
+ * into the pud entry)
+ */
+static inline int pud_none(pud_t pud)		{ return 0; }
+static inline int pud_bad(pud_t pud)		{ return 0; }
+static inline int pud_present(pud_t pud)	{ return 1; }
+static inline int pud_user(pud_t pud)		{ return 0; }
+static inline int pud_leaf(pud_t pud)		{ return 0; }
+static inline void pud_clear(pud_t *pud)	{ }
+#define pmd_ERROR(pmd)				(pud_ERROR((pmd).pud))
+
+#define pud_populate(mm, pmd, pte)		do { } while (0)
+
+/*
+ * (pmds are folded into puds so this doesn't get actually called,
+ * but the define is needed for a generic inline function.)
+ */
+#define set_pud(pudptr, pudval)			set_pmd((pmd_t *)(pudptr), (pmd_t) { pudval })
+
+static inline pmd_t * pmd_offset(pud_t * pud, unsigned long address)
+{
+	return (pmd_t *)pud;
+}
+#define pmd_offset pmd_offset
+
+#define pmd_val(x)				(pud_val((x).pud))
+#define __pmd(x)				((pmd_t) { __pud(x) } )
+
+#define pud_page(pud)				(pmd_page((pmd_t){ pud }))
+#define pud_pgtable(pud)			((pmd_t *)(pmd_page_vaddr((pmd_t){ pud })))
+
+/*
+ * allocating and freeing a pmd is trivial: the 1-entry pmd is
+ * inside the pud, so has no extra memory associated with it.
+ */
+#define pmd_alloc_one(mm, address)		NULL
+static inline void pmd_free(struct mm_struct *mm, pmd_t *pmd)
+{
+}
+#define pmd_free_tlb(tlb, x, a)		do { } while (0)
+
+#undef  pmd_addr_end
+#define pmd_addr_end(addr, end)			(end)
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _PGTABLE_NOPMD_H */