x86/kasan: Populate shadow for shared chunk of the CPU entry area

Popuplate the shadow for the shared portion of the CPU entry area, i.e.
the read-only IDT mapping, during KASAN initialization.  A recent change
modified KASAN to map the per-CPU areas on-demand, but forgot to keep a
shadow for the common area that is shared amongst all CPUs.

Map the common area in KASAN init instead of letting idt_map_in_cea() do
the dirty work so that it Just Works in the unlikely event more shared
data is shoved into the CPU entry area.

The bug manifests as a not-present #PF when software attempts to lookup
an IDT entry, e.g. when KVM is handling IRQs on Intel CPUs (KVM performs
direct CALL to the IRQ handler to avoid the overhead of INTn):

 BUG: unable to handle page fault for address: fffffbc0000001d8
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 16c03a067 P4D 16c03a067 PUD 0
 Oops: 0000 [#1] PREEMPT SMP KASAN
 CPU: 5 PID: 901 Comm: repro Tainted: G        W          6.1.0-rc3+ #410
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
 RIP: 0010:kasan_check_range+0xdf/0x190
  vmx_handle_exit_irqoff+0x152/0x290 [kvm_intel]
  vcpu_run+0x1d89/0x2bd0 [kvm]
  kvm_arch_vcpu_ioctl_run+0x3ce/0xa70 [kvm]
  kvm_vcpu_ioctl+0x349/0x900 [kvm]
  __x64_sys_ioctl+0xb8/0xf0
  do_syscall_64+0x2b/0x50
  entry_SYSCALL_64_after_hwframe+0x46/0xb0

Fixes: 9fd429c28073 ("x86/kasan: Map shadow for percpu pages on demand")
Reported-by: syzbot+8cdd16fd5a6c0565e227@syzkaller.appspotmail.com
Signed-off-by: NSean Christopherson <seanjc@google.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221110203504.1985010-6-seanjc@google.com

arch/x86/mm/kasan_init_64.c

@@ -341,7 +341,7 @@ void __init kasan_populate_shadow_for_vaddr(void *va, size_t size, int nid)
 
 void __init kasan_init(void)
 {
-	unsigned long shadow_cea_begin, shadow_cea_end;
+	unsigned long shadow_cea_begin, shadow_cea_per_cpu_begin, shadow_cea_end;
 	int i;
 
 	memcpy(early_top_pgt, init_top_pgt, sizeof(early_top_pgt));
@@ -384,6 +384,7 @@ void __init kasan_init(void)
 	}
 
 	shadow_cea_begin = kasan_mem_to_shadow_align_down(CPU_ENTRY_AREA_BASE);
+	shadow_cea_per_cpu_begin = kasan_mem_to_shadow_align_up(CPU_ENTRY_AREA_PER_CPU);
 	shadow_cea_end = kasan_mem_to_shadow_align_up(CPU_ENTRY_AREA_BASE +
 						      CPU_ENTRY_AREA_MAP_SIZE);
 
@@ -409,6 +410,15 @@ void __init kasan_init(void)
 		kasan_mem_to_shadow((void *)VMALLOC_END + 1),
 		(void *)shadow_cea_begin);
 
+	/*
+	 * Populate the shadow for the shared portion of the CPU entry area.
+	 * Shadows for the per-CPU areas are mapped on-demand, as each CPU's
+	 * area is randomly placed somewhere in the 512GiB range and mapping
+	 * the entire 512GiB range is prohibitively expensive.
+	 */
+	kasan_populate_shadow(shadow_cea_begin,
+			      shadow_cea_per_cpu_begin, 0);
+
 	kasan_populate_early_shadow((void *)shadow_cea_end,
 			kasan_mem_to_shadow((void *)__START_KERNEL_map));