Populate a KASAN shadow for the entire possible per-CPU range of the CPU
entry area instead of requiring that each individual chunk map a shadow.
Mapping shadows individually is error prone, e.g. the per-CPU GDT mapping
was left behind, which can lead to not-present page faults during KASAN
validation if the kernel performs a software lookup into the GDT.  The DS
buffer is also likely affected.

The motivation for mapping the per-CPU areas on-demand was to avoid
mapping the entire 512GiB range that's reserved for the CPU entry area,
shaving a few bytes by not creating shadows for potentially unused memory
was not a goal.

The bug is most easily reproduced by doing a sigreturn with a garbage
CS in the sigcontext, e.g.

  int main(void)
  {
    struct sigcontext regs;

    syscall(__NR_mmap, 0x1ffff000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
    syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 7ul, 0x32ul, -1, 0ul);
    syscall(__NR_mmap, 0x21000000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);

    memset(&regs, 0, sizeof(regs));
    regs.cs = 0x1d0;
    syscall(__NR_rt_sigreturn);
    return 0;
  }

to coerce the kernel into doing a GDT lookup to compute CS.base when
reading the instruction bytes on the subsequent #GP to determine whether
or not the #GP is something the kernel should handle, e.g. to fixup UMIP
violations or to emulate CLI/STI for IOPL=3 applications.

  BUG: unable to handle page fault for address: fffffbc8379ace00
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 16c03a067 P4D 16c03a067 PUD 15b990067 PMD 15b98f067 PTE 0
  Oops: 0000 [#1] PREEMPT SMP KASAN
  CPU: 3 PID: 851 Comm: r2 Not tainted 6.1.0-rc3-next-20221103+ #432
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:kasan_check_range+0xdf/0x190
  Call Trace:
   <TASK>
   get_desc+0xb0/0x1d0
   insn_get_seg_base+0x104/0x270
   insn_fetch_from_user+0x66/0x80
   fixup_umip_exception+0xb1/0x530
   exc_general_protection+0x181/0x210
   asm_exc_general_protection+0x22/0x30
  RIP: 0003:0x0
  Code: Unable to access opcode bytes at 0xffffffffffffffd6.
  RSP: 0003:0000000000000000 EFLAGS: 00000202
  RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00000000000001d0
  RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
  RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
  R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
   </TASK>

Fixes: 9fd429c28073 ("x86/kasan: Map shadow for percpu pages on demand")
Reported-by: syzbot+ffb4f000dc2872c93f62@syzkaller.appspotmail.com
Suggested-by: NAndrey Ryabinin <ryabinin.a.a@gmail.com>
Signed-off-by: NSean Christopherson <seanjc@google.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: NAndrey Ryabinin <ryabinin.a.a@gmail.com>
Link: https://lkml.kernel.org/r/20221110203504.1985010-3-seanjc@google.com