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/arch/x86/boot/apm.c b/arch/x86/boot/apm.c
new file mode 100644
index 000000000..bda15f967
--- /dev/null
+++ b/arch/x86/boot/apm.c
@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* -*- linux-c -*- ------------------------------------------------------- *
+ *
+ *   Copyright (C) 1991, 1992 Linus Torvalds
+ *   Copyright 2007 rPath, Inc. - All Rights Reserved
+ *   Copyright 2009 Intel Corporation; author H. Peter Anvin
+ *
+ *   Original APM BIOS checking by Stephen Rothwell, May 1994
+ *   (sfr@canb.auug.org.au)
+ *
+ * ----------------------------------------------------------------------- */
+
+/*
+ * Get APM BIOS information
+ */
+
+#include "boot.h"
+
+int query_apm_bios(void)
+{
+	struct biosregs ireg, oreg;
+
+	/* APM BIOS installation check */
+	initregs(&ireg);
+	ireg.ah = 0x53;
+	intcall(0x15, &ireg, &oreg);
+
+	if (oreg.flags & X86_EFLAGS_CF)
+		return -1;		/* No APM BIOS */
+
+	if (oreg.bx != 0x504d)		/* "PM" signature */
+		return -1;
+
+	if (!(oreg.cx & 0x02))		/* 32 bits supported? */
+		return -1;
+
+	/* Disconnect first, just in case */
+	ireg.al = 0x04;
+	intcall(0x15, &ireg, NULL);
+
+	/* 32-bit connect */
+	ireg.al = 0x03;
+	intcall(0x15, &ireg, &oreg);
+
+	boot_params.apm_bios_info.cseg        = oreg.ax;
+	boot_params.apm_bios_info.offset      = oreg.ebx;
+	boot_params.apm_bios_info.cseg_16     = oreg.cx;
+	boot_params.apm_bios_info.dseg        = oreg.dx;
+	boot_params.apm_bios_info.cseg_len    = oreg.si;
+	boot_params.apm_bios_info.cseg_16_len = oreg.hsi;
+	boot_params.apm_bios_info.dseg_len    = oreg.di;
+
+	if (oreg.flags & X86_EFLAGS_CF)
+		return -1;
+
+	/* Redo the installation check as the 32-bit connect;
+	   some BIOSes return different flags this way... */
+
+	ireg.al = 0x00;
+	intcall(0x15, &ireg, &oreg);
+
+	if ((oreg.eflags & X86_EFLAGS_CF) || oreg.bx != 0x504d) {
+		/* Failure with 32-bit connect, try to disconnect and ignore */
+		ireg.al = 0x04;
+		intcall(0x15, &ireg, NULL);
+		return -1;
+	}
+
+	boot_params.apm_bios_info.version = oreg.ax;
+	boot_params.apm_bios_info.flags   = oreg.cx;
+	return 0;
+}
+