x86/alternatives: Fix int3_emulate_call() selftest stack corruption

KASAN shows the following splat during boot:

  BUG: KASAN: unknown-crash in unwind_next_frame+0x3f6/0x490
  Read of size 8 at addr ffffffff84007db0 by task swapper/0

  CPU: 0 PID: 0 Comm: swapper Tainted: G                T 5.2.0-rc6-00013-g7457c0da #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
  Call Trace:
   dump_stack+0x19/0x1b
   print_address_description+0x1b0/0x2b2
   __kasan_report+0x10f/0x171
   kasan_report+0x12/0x1c
   __asan_load8+0x54/0x81
   unwind_next_frame+0x3f6/0x490
   unwind_next_frame+0x1b/0x23
   arch_stack_walk+0x68/0xa5
   stack_trace_save+0x7b/0xa0
   save_trace+0x3c/0x93
   mark_lock+0x1ef/0x9b1
   lock_acquire+0x122/0x221
   __mutex_lock+0xb6/0x731
   mutex_lock_nested+0x16/0x18
   _vm_unmap_aliases+0x141/0x183
   vm_unmap_aliases+0x14/0x16
   change_page_attr_set_clr+0x15e/0x2f2
   set_memory_4k+0x2a/0x2c
   check_bugs+0x11fd/0x1298
   start_kernel+0x793/0x7eb
   x86_64_start_reservations+0x55/0x76
   x86_64_start_kernel+0x87/0xaa
   secondary_startup_64+0xa4/0xb0

  Memory state around the buggy address:
   ffffffff84007c80: 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1
   ffffffff84007d00: f1 00 00 00 00 00 00 00 00 00 f2 f2 f2 f3 f3 f3
  >ffffffff84007d80: f3 79 be 52 49 79 be 00 00 00 00 00 00 00 00 f1

It turns out that int3_selftest() is corrupting the stack.  The problem is
that the KASAN-ified version of int3_magic() is much less trivial than the
C code appears.  It clobbers several unexpected registers.  So when the
selftest's INT3 is converted to an emulated call to int3_magic(), the
registers are clobbered and Bad Things happen when the function returns.

Fix this by converting int3_magic() to the trivial ASM function it should
be, avoiding all calling convention issues. Also add ASM_CALL_CONSTRAINT to
the INT3 ASM, since it contains a 'CALL'.

[peterz: cribbed changelog from josh]

Fixes: 7457c0da ("x86/alternatives: Add int3_emulate_call() selftest")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Debugged-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20190709125744.GB3402@hirez.programming.kicks-ass.net

arch/x86/kernel/alternative.c

@@ -625,10 +625,23 @@ extern struct paravirt_patch_site __start_parainstructions[],
  *
  * See entry_{32,64}.S for more details.
  */
-static void __init int3_magic(unsigned int *ptr)
-{
-	*ptr = 1;
-}
+
+/*
+ * We define the int3_magic() function in assembly to control the calling
+ * convention such that we can 'call' it from assembly.
+ */
+
+extern void int3_magic(unsigned int *ptr); /* defined in asm */
+
+asm (
+"	.pushsection	.init.text, \"ax\", @progbits\n"
+"	.type		int3_magic, @function\n"
+"int3_magic:\n"
+"	movl	$1, (%" _ASM_ARG1 ")\n"
+"	ret\n"
+"	.size		int3_magic, .-int3_magic\n"
+"	.popsection\n"
+);
 
 extern __initdata unsigned long int3_selftest_ip; /* defined in asm below */
 
@@ -676,7 +689,9 @@ static void __init int3_selftest(void)
 		      "int3_selftest_ip:\n\t"
 		      __ASM_SEL(.long, .quad) " 1b\n\t"
 		      ".popsection\n\t"
-		      : : __ASM_SEL_RAW(a, D) (&val) : "memory");
+		      : ASM_CALL_CONSTRAINT
+		      : __ASM_SEL_RAW(a, D) (&val)
+		      : "memory");
 
 	BUG_ON(val != 1);