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Subject: Re: [patch V2 28/29] x86/irq/64: Remap the IRQ stack with guard pages
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1904070803200.1840@nanos.tec.linutronix.de>
Date:   Sun, 7 Apr 2019 02:28:07 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>
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Message-Id: <50F7A079-11F5-45EA-B378-7527B5920A7C@amacapital.net>
References: <20190405150658.237064784@linutronix.de> <20190405150930.967389183@linutronix.de> <CALCETrWF_YgQODJ=Rpm2MnhFxJXCfoo4H792hUQni0GS9on7og@mail.gmail.com> <alpine.DEB.2.21.1904070803200.1840@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
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> On Apr 6, 2019, at 11:08 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
>> On Sat, 6 Apr 2019, Andy Lutomirski wrote:
>>> On Fri, Apr 5, 2019 at 8:11 AM Thomas Gleixner <tglx@linutronix.de> wrot=
e:
>>>=20
>>> From: Andy Lutomirski <luto@kernel.org>
>>>=20
>>> The IRQ stack lives in percpu space, so an IRQ handler that overflows it=

>>> will overwrite other data structures.
>>>=20
>>> Use vmap() to remap the IRQ stack so that it will have the usual guard
>>> pages that vmap/vmalloc allocations have. With this the kernel will pani=
c
>>> immediately on an IRQ stack overflow.
>>=20
>> The 0day bot noticed that this dies with DEBUG_PAGEALLOC on.  This is
>> because the store_stackinfo() function is utter garbage and this patch
>> correctly detects just how broken it is.  The attached patch "fixes"
>> it.  (It also contains a reliability improvement that should probably
>> get folded in, but is otherwise unrelated.)
>>=20
>> A real fix would remove the generic kstack_end() function entirely
>> along with __HAVE_ARCH_KSTACK_END and would optionally replace
>> store_stackinfo() with something useful.  Josh, do we have a generic
>> API to do a little stack walk like this?  Otherwise, I don't think it
>> would be the end of the world to just remove the offending code.
>=20
> Yes, I found the same yesterday before heading out. It's already broken
> with the percpu stack because there is no guarantee that the per cpu stack=

> is thread size aligned. It's guaranteed to be page aligned not more.
>=20
> I'm all for removing that nonsense, but the real question is whether there=

> is more code which assumes THREAD_SIZE aligned stacks aside of the thread
> stack itself.
>=20
>=20

Well, any code like this is already busted, since the stacks alignment doesn=
=E2=80=99t really change with these patches applied.=