kmod/core: fix crash with !CONFIG_DEBUG_SET_MODULE_RONX

When loading a patch module on a kernel with
!CONFIG_DEBUG_SET_MODULE_RONX, the following crash occurs:

  loading core module: /root/src/kpatch/kpatch/../kmod/core/kpatch.ko
  loading patch module: kpatch-meminfo-string.ko
  BUG: unable to handle kernel paging request at ffffffffa0010cc0
  IP: [<ffffffff8125ecb0>] do_init_module+0x84/0x1af
  PGD 13d3067 PUD 13d4063 PMD 1e1ee067 PTE 1e1a0161
  Oops: 0003 [dynup#1]
  Modules linked in: kpatch_meminfo_string(O+) kpatch(O)
  CPU: 0 PID: 149 Comm: insmod Tainted: G           O  K 4.1.0+ dynup#1
  task: ffff88001e17b810 ti: ffff88001e1cc000 task.ti: ffff88001e1cc000
  RIP: 0010:[<ffffffff8125ecb0>]  [<ffffffff8125ecb0>] do_init_module+0x84/0x1af
  RSP: 0018:ffff88001e1cfda8  EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffffffffa0010cc0 RCX: 0000000080a02001
  RDX: 0000000000000024 RSI: 0000000000000000 RDI: ffffffff813fabe0
  RBP: 0000000000000000 R08: 0000000000000000 R09: 00000000d0000000
  R10: ffffffffa000e000 R11: 0000000000000001 R12: ffff88001eb58638
  R13: ffffffffa0010d10 R14: 0000000000000001 R15: 0000000000000000
  FS:  00007f0ae00aa700(0000) GS:ffffffff813e1000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
  CR2: ffffffffa0010cc0 CR3: 000000001e181000 CR4: 00000000000006b0
  Stack:
   ffff88001e1cfed8 0000000000000001 ffffffffa0010cc0 ffffffff81058aac
   ffff88001e207680 00000000810a462f ffffc90000096890 0000000000000e00
   ffffffff00000016 ffffffff8126cd40 ffff88001eaa6a08 ffff88001e1cfe48
  Call Trace:
   [<ffffffff81058aac>] ? load_module+0x18ad/0x18e9
   [<ffffffff81056290>] ? copy_module_from_fd+0x86/0xdf
   [<ffffffff81058c1e>] ? SyS_finit_module+0x56/0x61
   [<ffffffff81261854>] ? system_call_fastpath+0x12/0x6a
  Code: f8 00 00 00 74 23 49 c7 c0 80 ca 26 81 48 8d 53 18 89 c1 4c 89 c6 48 c7 c7 6d ef 36 81 31 c0 e8 16 fb ff ff e8 18 06 00 00 31 f6 <c7> 03 00 00 00 00 48 89 da 48 c7 c7 c0 c9 3f 81 e8 7e b3 dd ff
  RIP  [<ffffffff8125ecb0>] do_init_module+0x84/0x1af
   RSP <ffff88001e1cfda8>
  CR2: ffffffffa0010cc0

With !CONFIG_DEBUG_SET_MODULE_RONX, module text and rodata pages are
writable, and the debug_align() macro allows the module struct to share
a page with executable text.  When klp_write_module_reloc() calls
set_memory_ro() on the page, it effectively turns the module struct into
a read-only structure, resulting in a page fault when load_module() does
"mod->state = MODULE_STATE_LIVE".

Fixes: dynup#497

kmod/core/core.c

@@ -584,7 +584,6 @@ static int kpatch_write_relocations(struct kpatch_module *kpmod,
 	struct kpatch_dynrela *dynrela;
 	u64 loc, val;
 	unsigned long core = (unsigned long)kpmod->mod->module_core;
-	unsigned long core_ro_size = kpmod->mod->core_ro_size;
 	unsigned long core_size = kpmod->mod->core_size;
 	unsigned long src;
 
@@ -639,16 +638,17 @@ static int kpatch_write_relocations(struct kpatch_module *kpmod,
 			return -EINVAL;
 		}
 
-		if (loc >= core && loc < core + core_ro_size)
-			readonly = 1;
-		else if (loc >= core + core_ro_size && loc < core + core_size)
-			readonly = 0;
-		else {
+		if (loc < core || loc >= core + core_size) {
 			pr_err("bad dynrela location 0x%llx for symbol %s\n",
 			       loc, dynrela->name);
 			return -EINVAL;
 		}
 
+#ifdef CONFIG_DEBUG_SET_MODULE_RONX
+		if (loc < core + kpmod->mod->core_ro_size)
+			readonly = 1;
+#endif
+
 		numpages = (PAGE_SIZE - (loc & ~PAGE_MASK) >= size) ? 1 : 2;
 
 		if (readonly)