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Date:   Sun, 7 Apr 2019 11:23:38 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@kernel.org>
cc:     LKML <linux-kernel@vger.kernel.org>
Subject: Re: 8b275b3754 ("x86/irq/64: Remap the IRQ stack with guard pages"):
 BUG: unable to handle kernel paging request at ffffb659000a1000
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On Sat, 6 Apr 2019, Andy Lutomirski wrote:
> I haven't spotted the actual bug yet, but the faulting instruction is:
> 
>   2a:    65 8b 35 09 ca 75 63     mov    %gs:*0x6375ca09(%rip),%esi
>     # 0x6375ca3a        <-- trapping instruction
> 
> This seems to be faulting just above the top of the stack (the thing
> in RSP), so I suspect that there is some path that is shoving the
> remapped value into GSBASE, which is wrong.
> 
> Also, FWIW, there was some reason that I initialized all the virtual
> mappings for all possible CPUs early.  I don't remember what it was,
> and it may not have been a good reason, but I put at least some
> nonzero amount of thought into it :)

There is absolutely no reason to have irq stacks before init_IRQ(). 32bit
uses at runtime allocated irq stacks for years.

If the CPU takes a device interrupt before that, then there are way more
things which explode than just the irqstack pointer being NULL.

Thanks,

	tglx