x86/dumpstack: Add show_stack_regs() and use it

Add a helper to dump supplied pt_regs and use it in the MSR exception
handling code to have precise stack traces pointing to the actual
function causing the MSR access exception and not the stack frame of the
exception handler itself.

The new output looks like this:

 unchecked MSR access error: RDMSR from 0xdeadbeef at rIP: 0xffffffff8102ddb6 (early_init_intel+0x16/0x3a0)
  00000000756e6547 ffffffff81c03f68 ffffffff81dd0940 ffffffff81c03f10
  ffffffff81d42e65 0000000001000000 ffffffff81c03f58 ffffffff81d3e5a3
  0000800000000000 ffffffff81800080 ffffffffffffffff 0000000000000000
 Call Trace:
  [<ffffffff81d42e65>] early_cpu_init+0xe7/0x136
  [<ffffffff81d3e5a3>] setup_arch+0xa5/0x9df
  [<ffffffff81d38bb9>] start_kernel+0x9f/0x43a
  [<ffffffff81d38294>] x86_64_start_reservations+0x2f/0x31
  [<ffffffff81d383fe>] x86_64_start_kernel+0x168/0x176

Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1467671487-10344-4-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/kdebug.h

@@ -26,6 +26,7 @@ extern void die(const char *, struct pt_regs *,long);
 extern int __must_check __die(const char *, struct pt_regs *, long);
 extern void show_trace(struct task_struct *t, struct pt_regs *regs,
 		       unsigned long *sp, unsigned long bp);
+extern void show_stack_regs(struct pt_regs *regs);
 extern void __show_regs(struct pt_regs *regs, int all);
 extern unsigned long oops_begin(void);
 extern void oops_end(unsigned long, struct pt_regs *, int signr);

arch/x86/kernel/dumpstack.c

@@ -197,6 +197,11 @@ void show_stack(struct task_struct *task, unsigned long *sp)
 	show_stack_log_lvl(task, NULL, sp, bp, "");
 }
 
+void show_stack_regs(struct pt_regs *regs)
+{
+	show_stack_log_lvl(current, regs, (unsigned long *)regs->sp, regs->bp, "");
+}
+
 static arch_spinlock_t die_lock = __ARCH_SPIN_LOCK_UNLOCKED;
 static int die_owner = -1;
 static unsigned int die_nest_count;

arch/x86/mm/extable.c

@@ -1,6 +1,7 @@
 #include <linux/module.h>
 #include <asm/uaccess.h>
 #include <asm/traps.h>
+#include <asm/kdebug.h>
 
 typedef bool (*ex_handler_t)(const struct exception_table_entry *,
 			    struct pt_regs *, int);
@@ -46,8 +47,9 @@ EXPORT_SYMBOL(ex_handler_ext);
 bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
 			     struct pt_regs *regs, int trapnr)
 {
-	WARN_ONCE(1, "unchecked MSR access error: RDMSR from 0x%x\n",
-		  (unsigned int)regs->cx);
+	if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pF)\n",
+			 (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
+		show_stack_regs(regs);
 
 	/* Pretend that the read succeeded and returned 0. */
 	regs->ip = ex_fixup_addr(fixup);
@@ -60,9 +62,10 @@ EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
 bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
 			     struct pt_regs *regs, int trapnr)
 {
-	WARN_ONCE(1, "unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x)\n",
-		  (unsigned int)regs->cx,
-		  (unsigned int)regs->dx, (unsigned int)regs->ax);
+	if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pF)\n",
+			 (unsigned int)regs->cx, (unsigned int)regs->dx,
+			 (unsigned int)regs->ax,  regs->ip, (void *)regs->ip))
+		show_stack_regs(regs);
 
 	/* Pretend that the write succeeded. */
 	regs->ip = ex_fixup_addr(fixup);