Re: [PATCH 11/16] x86/asm/64: Use a percpu trampoline stack for IDT entries

[Andy Lutomirski]

On Mon, Nov 20, 2017 at 11:20 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> index 1ea03027a4a9..e4a941be96cf 100644
>> --- a/arch/x86/kernel/traps.c
>> +++ b/arch/x86/kernel/traps.c
>
>> -asmlinkage __visible notrace
>> +asmlinkage __visible notrace __no_sanitize_address
>>  struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
>>  {
>>       /*
>>        * This is called from entry_64.S early in handling a fault
>>        * caused by a bad iret to user mode.  To handle the fault
>> -      * correctly, we want move our stack frame to task_pt_regs
>> -      * and we want to pretend that the exception came from the
>> -      * iret target.
>> +      * correctly, we want move our stack frame to where it would
>> +      * be had we entered directly on the entry stack (rather than
>> +      * just below the IRET frame) and we want to pretend that the
>> +      * exception came from the iret target.
>>        */
>>       struct bad_iret_stack *new_stack =
>> -             container_of(task_pt_regs(current),
>> -                          struct bad_iret_stack, regs);
>> +             (struct bad_iret_stack *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;
>>
>>       /* Copy the IRET target to the new stack. */
>>       memmove(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);
>
> So the addition of the __no_sanitize_address attribute to fixup_bad_iret() made
> the 64-bit allyesconfig/allmodconfig kernels fail the build:

Yeah, I *hate* the way this works.  Apparently getting
__no_sanitize_address to be fully reliable involves just building the
whole file without KASAN.

That being said, I have this fixed much better in my latest tree: I
fixed up KASAN so that I don't need to turn it off for these helpers.