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, 37 insertions, 0 deletions

diff --git a/arch/hexagon/mm/uaccess.c b/arch/hexagon/mm/uaccess.c
new file mode 100644
index 000000000..3204e9ba6
--- /dev/null
+++ b/arch/hexagon/mm/uaccess.c
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2010-2011, The Linux Foundation. All rights reserved.
+ */
+
+/*
+ * Support for user memory access from kernel.  This will
+ * probably be inlined for performance at some point, but
+ * for ease of debug, and to a lesser degree for code size,
+ * we implement here as subroutines.
+ */
+#include <linux/types.h>
+#include <linux/uaccess.h>
+#include <linux/pgtable.h>
+
+/*
+ * For clear_user(), exploit previously defined copy_to_user function
+ * and the fact that we've got a handy zero page defined in kernel/head.S
+ *
+ * dczero here would be even faster.
+ */
+__kernel_size_t __clear_user_hexagon(void __user *dest, unsigned long count)
+{
+	long uncleared;
+
+	while (count > PAGE_SIZE) {
+		uncleared = raw_copy_to_user(dest, &empty_zero_page, PAGE_SIZE);
+		if (uncleared)
+			return count - (PAGE_SIZE - uncleared);
+		count -= PAGE_SIZE;
+		dest += PAGE_SIZE;
+	}
+	if (count)
+		count = raw_copy_to_user(dest, &empty_zero_page, count);
+
+	return count;
+}