Re: PROBLEM: consolidated IDT invalidation causes kexec to reboot

[hpa]

On December 26, 2017 10:51:12 AM PST, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>[ Sorry, I was off-line on Christmas Eve due to festivities, and then
>yesterday because I've apparently caught a cold.
>
>  Still not back to normal, but at least I can sit in front of the
>computer again ]
>
>On Mon, Dec 25, 2017 at 1:29 PM, Alexandru Chirvasitu
><achirvasub@xxxxxxxxx> wrote:
>>
>> On Mon, Dec 25, 2017 at 06:40:14AM -0800, Andy Lutomirski wrote:
>>>
>>> This is presumably the same call-tracing-without-TLS-working
>problem.
>>> idt_invalidate() is out-of-line and is compiled with full tracing
>on,
>
>Yeah. The difference literally seems to be that in the old case it was
>accidentally inlined.
>
>I say "accidentally", because "load_idt()" itself is explicitly
>inlined, but the "set_idt()" in machine_kexec_32.c was not.
>
>But before that commit ("e802a51ede91 x86/idt: Consolidate IDT
>invalidation") the compiler inlined it anyway because it was a small
>static function.
>
>Afterwards, not so much (different C file), and then the stack tracing
>code blew up because of the incomplete CPU state.
>
>> This works.. I went back to the troublesome commit e802a51 and
>> modified it as follows:
>>
>> +/**
>> + * idt_invalidate - Invalidate interrupt descriptor table
>> + * @addr:       The virtual address of the 'invalid' IDT
>> + */
>> +static inline void idt_invalidate(void *addr)
>> +{
>> +        struct desc_ptr idt = { .address = (unsigned long) addr,
>.size = 0 };
>> +
>> +        load_idt(&idt);
>> +}
>
>Yes, I suspect that is the right thing to do. It's small enough that
>inliningh it makes sense.
>
>HOWEVER. Would you mind testing a totally different fix instead?
>
>In particular, take the current top of tree (that doesn't work for
>you), and try to just change the order of these two lines:
>
>        set_gdt(phys_to_virt(0), 0);
>        idt_invalidate(phys_to_virt(0));
>
>in arch/x86/kernel/machine_kexec_32.c.
>
>I think it's a better idea to invalidate the IDT first, because that
>is only used for exceptions. In contrast, invalidating the GDT will
>obviously make any segment load do horrible things, _and_ any
>exceptions would fail anyway (because exceptions need segments too).
>
>So in many ways, that "set_get()" that invalidates the GDT is the more
>destructive thing, and should be done last.
>
>And if we do it last, maybe the whole "oops, we have tracing code
>enabled" thing wouldn't have mattered.
>
>Does that trivial line switching make the old broken config work for
>you again?
>
>> kexec now works as expected; tested repeatedly, both with direct
>> execution and crash triggering.
>>
>> I had to google 'inline function' :)).
>
>We'll make a kernel developer out of you yet. You've already found the
>most important development tool (I kid, I kid. Google is useful, but
>"willingness to try things out" is actually the #1 thing).
>
>Mind googling "linux kernel patch submission" and adding the required
>sign-off, and I suspect the x86 people will happily take your patch?
>
>That said, I do wonder about a few things:
>
> - the 'addr' argument is pointless, afaik. I *suspect* it used to be
>0, and then some mindless editing just changed it to that
>"phys_to_virt(0)".
>
>   With a zero length, it shouldn't matter what the actual IDT base
>address actually is. Any access is going to trap regardless.
>
> - some people were clearly aware of just how critical that whole
>"load_idt()" sequence were, because things were marked "inline" and
>"NOKPROBE_SYMBOL()" etc, but there was no comment in the code that
>actually did this about how the machine state is total garbage after
>the "set_gdt()" in machine_kexec().
>
> - the above "I think we should invalidate GDT last" issue.
>
>Hmm?
>
>                  Linus

Can anyone explain why on Earth we do a phys_to_virt() instead of just stuffing in a zero, since this is a null pointer anyway?!

The other option would be to use the real mode IDT settings (address 0, limit 0xffff although 0x3ff is equivalent.)
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