x86/traps: avoid KMSAN bugs originating from handle_bug()

There is a case in exc_invalid_op handler that is executed outside the
irqentry_enter()/irqentry_exit() region when an UD2 instruction is used to
encode a call to __warn().

In that case the `struct pt_regs` passed to the interrupt handler is never
unpoisoned by KMSAN (this is normally done in irqentry_enter()), which
leads to false positives inside handle_bug().

Use kmsan_unpoison_entry_regs() to explicitly unpoison those registers
before using them.

Link: https://lkml.kernel.org/r/20221102110611.1085175-5-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Marco Elver <elver@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/x86/kernel/traps.c

@@ -15,6 +15,7 @@
 #include <linux/context_tracking.h>
 #include <linux/interrupt.h>
 #include <linux/kallsyms.h>
+#include <linux/kmsan.h>
 #include <linux/spinlock.h>
 #include <linux/kprobes.h>
 #include <linux/uaccess.h>
@@ -301,6 +302,12 @@ static noinstr bool handle_bug(struct pt_regs *regs)
 {
 	bool handled = false;
 
+	/*
+	 * Normally @regs are unpoisoned by irqentry_enter(), but handle_bug()
+	 * is a rare case that uses @regs without passing them to
+	 * irqentry_enter().
+	 */
+	kmsan_unpoison_entry_regs(regs);
 	if (!is_valid_bugaddr(regs->ip))
 		return handled;