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Subject: Re: 8b275b3754 ("x86/irq/64: Remap the IRQ stack with guard pages"): BUG: unable to handle kernel paging request at ffffb659000a1000
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1904071121270.1840@nanos.tec.linutronix.de>
Date:   Sun, 7 Apr 2019 02:26:54 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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Message-Id: <A03AEEC0-2398-42DE-80DF-7BE4D81EFEF5@amacapital.net>
References: <5ca84952.hpEqUp6kap0tafzR%lkp@intel.com> <CALCETrX_ys9dJZOkN7QBjWz9Y1iukQDaJaASPAkGtWvwNkOESw@mail.gmail.com> <alpine.DEB.2.21.1904071121270.1840@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
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> On Apr 7, 2019, at 2:23 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
>> On Sat, 6 Apr 2019, Andy Lutomirski wrote:
>> I haven't spotted the actual bug yet, but the faulting instruction is:
>>=20
>>  2a:    65 8b 35 09 ca 75 63     mov    %gs:*0x6375ca09(%rip),%esi
>>    # 0x6375ca3a        <-- trapping instruction
>>=20
>> 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.
>>=20
>> 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 :)
>=20
> There is absolutely no reason to have irq stacks before init_IRQ(). 32bit
> uses at runtime allocated irq stacks for years.
>=20
> If the CPU takes a device interrupt before that, then there are way more
> things which explode than just the irqstack pointer being NULL.
>=20

Fair enough.  Although the patch I emailed in the other thread allows at lea=
st 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 t=
o disregard that part :)=