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, 61 insertions, 0 deletions

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
new file mode 100644
index 000000000..14cd43d4d
--- /dev/null
+++ b/arch/x86/entry/common.c
@@ -0,0 +1,61 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/entry-common.h>
+#include <linux/kvm_types.h>
+#include <linux/hrtimer_rearm.h>
+#include <asm/fred.h>
+#include <asm/desc.h>
+
+#if IS_ENABLED(CONFIG_KVM_INTEL)
+/*
+ * On VMX, NMIs and IRQs (as configured by KVM) are acknowledged by hardware as
+ * part of the VM-Exit, i.e. the event itself is consumed as part the VM-Exit.
+ * x86_entry_from_kvm() is invoked by KVM to effectively forward NMIs and IRQs
+ * to the kernel for servicing.  On SVM, a.k.a. AMD, the NMI/IRQ VM-Exit is
+ * purely a signal that an NMI/IRQ is pending, i.e. the event that triggered
+ * the VM-Exit is held pending until it's unblocked in the host.
+ */
+noinstr void x86_entry_from_kvm(unsigned int event_type, unsigned int vector)
+{
+	if (event_type == EVENT_TYPE_EXTINT) {
+#ifdef CONFIG_X86_64
+		/*
+		 * Use FRED dispatch, even when running IDT. The dispatch
+		 * tables are kept in sync between FRED and IDT, and the FRED
+		 * dispatch works well with CFI.
+		 */
+		fred_entry_from_kvm(event_type, vector);
+#else
+		idt_entry_from_kvm(vector);
+#endif
+		/*
+		 * Strictly speaking, only the NMI path requires noinstr.
+		 */
+		instrumentation_begin();
+		/*
+		 * KVM/VMX will dispatch from IRQ-disabled but for a context
+		 * that will have IRQs-enabled. This confuses the entry code
+		 * and it will not have reprogrammed the timer. Do so now.
+		 */
+		hrtimer_rearm_deferred();
+		instrumentation_end();
+
+		return;
+	}
+
+	WARN_ON_ONCE(event_type != EVENT_TYPE_NMI);
+
+#ifdef CONFIG_X86_64
+	if (cpu_feature_enabled(X86_FEATURE_FRED))
+		return fred_entry_from_kvm(event_type, vector);
+#endif
+
+	/*
+	 * Notably, we must use IDT dispatch for NMI when running in IDT mode.
+	 * The FRED NMI context is significantly different and will not work
+	 * right (specifically FRED fixed the NMI recursion issue).
+	 */
+	idt_do_nmi_irqoff();
+}
+EXPORT_SYMBOL_FOR_KVM(x86_entry_from_kvm);
+#endif