[PATCH 07/15] x86: Add int3_emulate_call() selftest

From: Peter Zijlstra <peterz@infradead.org>
To: x86@kernel.org
Cc: linux-kernel@vger.kernel.org,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Andy Lutomirski <luto@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
	David Laight <David.Laight@ACULAB.COM>,
	Borislav Petkov <bp@alien8.de>, Julia Cartwright <julia@ni.com>,
	Jessica Yu <jeyu@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Nadav Amit <namit@vmware.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Edward Cree <ecree@solarflare.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: [PATCH 07/15] x86: Add int3_emulate_call() selftest
Date: Wed, 05 Jun 2019 15:08:00 +0200	[thread overview]
Message-ID: <20190605131944.946978563@infradead.org> (raw)
In-Reply-To: 20190605130753.327195108@infradead.org

Given that the entry_*.S changes for this functionality are somewhat
tricky, make sure the paths are tested every boot, instead of on the
rare occasion when we trip an INT3 while rewriting text.

Requested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/alternative.c |   81 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 77 insertions(+), 4 deletions(-)

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -614,11 +614,83 @@ extern struct paravirt_patch_site __star
 	__stop_parainstructions[];
 #endif	/* CONFIG_PARAVIRT */
 
+/*
+ * Self-test for the INT3 based CALL emulation code.
+ *
+ * This exercises int3_emulate_call() to make sure INT3 pt_regs are set up
+ * properly and that there is a stack gap between the INT3 frame and the
+ * previous context. Without this gap doing a virtual PUSH on the interrupted
+ * stack would corrupt the INT3 IRET frame.
+ *
+ * See entry_{32,64}.S for more details.
+ */
+static void __init int3_magic(unsigned int *ptr)
+{
+	*ptr = 1;
+}
+
+extern __initdata unsigned long int3_selftest_ip; /* defined in asm below */
+
+static int __init
+int3_exception_notify(struct notifier_block *self, unsigned long val, void *data)
+{
+	struct die_args *args = data;
+	struct pt_regs *regs = args->regs;
+
+	if (!regs || user_mode(regs))
+		return NOTIFY_DONE;
+
+	if (val != DIE_INT3)
+		return NOTIFY_DONE;
+
+	if (regs->ip - INT3_INSN_SIZE != int3_selftest_ip)
+		return NOTIFY_DONE;
+
+	int3_emulate_call(regs, (unsigned long)&int3_magic);
+	return NOTIFY_STOP;
+}
+
+static void __init int3_selftest(void)
+{
+	static __initdata struct notifier_block int3_exception_nb = {
+		.notifier_call	= int3_exception_notify,
+		.priority	= INT_MAX-1, /* last */
+	};
+	unsigned int val = 0;
+
+	BUG_ON(register_die_notifier(&int3_exception_nb));
+
+	/*
+	 * Basically: int3_magic(&val); but really complicated :-)
+	 *
+	 * Stick the address of the INT3 instruction into int3_selftest_ip,
+	 * then trigger the INT3, padded with NOPs to match a CALL instruction
+	 * length.
+	 */
+	asm volatile ("1: int3; nop; nop; nop; nop\n\t"
+		      ".pushsection .init.data,\"aw\"\n\t"
+		      ".align " __ASM_SEL(4, 8) "\n\t"
+		      ".type int3_selftest_ip, @object\n\t"
+		      ".size int3_selftest_ip, " __ASM_SEL(4, 8) "\n\t"
+		      "int3_selftest_ip:\n\t"
+		      __ASM_SEL(.long, .quad) " 1b\n\t"
+		      ".popsection\n\t"
+		      : : __ASM_SEL_RAW(a, D) (&val) : "memory");
+
+	BUG_ON(val != 1);
+
+	unregister_die_notifier(&int3_exception_nb);
+}
+
 void __init alternative_instructions(void)
 {
-	/* The patching is not fully atomic, so try to avoid local interruptions
-	   that might execute the to be patched code.
-	   Other CPUs are not running. */
+	int3_selftest();
+
+	/*
+	 * The patching is not fully atomic, so try to avoid local
+	 * interruptions that might execute the to be patched code.
+	 * Other CPUs are not running.
+	 */
 	stop_nmi();
 
 	/*
@@ -643,10 +715,11 @@ void __init alternative_instructions(voi
 					    _text, _etext);
 	}
 
-	if (!uniproc_patched || num_possible_cpus() == 1)
+	if (!uniproc_patched || num_possible_cpus() == 1) {
 		free_init_pages("SMP alternatives",
 				(unsigned long)__smp_locks,
 				(unsigned long)__smp_locks_end);
+	}
 #endif
 
 	apply_paravirt(__parainstructions, __parainstructions_end);


