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, 65 insertions, 0 deletions

diff --git a/arch/x86/events/probe.c b/arch/x86/events/probe.c
new file mode 100644
index 000000000..bb719d0d3
--- /dev/null
+++ b/arch/x86/events/probe.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/export.h>
+#include <linux/types.h>
+#include <linux/bits.h>
+
+#include <asm/msr.h>
+#include "probe.h"
+
+static umode_t
+not_visible(struct kobject *kobj, struct attribute *attr, int i)
+{
+	return 0;
+}
+
+/*
+ * Accepts msr[] array with non populated entries as long as either
+ * msr[i].msr is 0 or msr[i].grp is NULL. Note that the default sysfs
+ * visibility is visible when group->is_visible callback is set.
+ */
+unsigned long
+perf_msr_probe(struct perf_msr *msr, int cnt, bool zero, void *data)
+{
+	unsigned long avail = 0;
+	unsigned int bit;
+	u64 val;
+
+	if (cnt >= BITS_PER_LONG)
+		return 0;
+
+	for (bit = 0; bit < cnt; bit++) {
+		if (!msr[bit].no_check) {
+			struct attribute_group *grp = msr[bit].grp;
+			u64 mask;
+
+			/* skip entry with no group */
+			if (!grp)
+				continue;
+
+			grp->is_visible = not_visible;
+
+			/* skip unpopulated entry */
+			if (!msr[bit].msr)
+				continue;
+
+			if (msr[bit].test && !msr[bit].test(bit, data))
+				continue;
+			/* Virt sucks; you cannot tell if a R/O MSR is present :/ */
+			if (rdmsrq_safe(msr[bit].msr, &val))
+				continue;
+
+			mask = msr[bit].mask;
+			if (!mask)
+				mask = ~0ULL;
+			/* Disable zero counters if requested. */
+			if (!zero && !(val & mask))
+				continue;
+
+			grp->is_visible = NULL;
+		}
+		avail |= BIT(bit);
+	}
+
+	return avail;
+}
+EXPORT_SYMBOL_GPL(perf_msr_probe);