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Date:   Sun, 7 Apr 2019 02:26:54 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: 8b275b3754 ("x86/irq/64: Remap the IRQ stack with guard pages"): BUG: unable to handle kernel paging request at ffffb659000a1000


> On Apr 7, 2019, at 2:23 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> 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.
> 

Fair enough.  Although the patch I emailed in the other thread allows at least the entry code to survive on 64-bit at the cost of just a couple lines of code.

But the kernel does indeed seem to work fine without the change. Feel free to disregard that part :)

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