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, 29 insertions, 0 deletions

diff --git a/arch/x86/um/fault.c b/arch/x86/um/fault.c
new file mode 100644
index 000000000..0dde4d613
--- /dev/null
+++ b/arch/x86/um/fault.c
@@ -0,0 +1,29 @@
+/* 
+ * Copyright (C) 2002 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
+ * Licensed under the GPL
+ */
+
+#include <arch.h>
+#include <sysdep/ptrace.h>
+
+/* These two are from asm-um/uaccess.h and linux/module.h, check them. */
+struct exception_table_entry
+{
+	unsigned long insn;
+	unsigned long fixup;
+};
+
+const struct exception_table_entry *search_exception_tables(unsigned long add);
+
+/* Compare this to arch/i386/mm/extable.c:fixup_exception() */
+int arch_fixup(unsigned long address, struct uml_pt_regs *regs)
+{
+	const struct exception_table_entry *fixup;
+
+	fixup = search_exception_tables(address);
+	if (fixup) {
+		UPT_IP(regs) = fixup->fixup;
+		return 1;
+	}
+	return 0;
+}