linux-kernel - Re: [patch V2 28/29] x86/irq/64: Remap the IRQ stack with guard pages

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Date:   Sun, 7 Apr 2019 07:03:41 -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>, X86 ML <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [patch V2 28/29] x86/irq/64: Remap the IRQ stack with guard pages


> On Apr 7, 2019, at 2:34 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> On Sun, 7 Apr 2019, Andy Lutomirski wrote:
>>> On Apr 6, 2019, at 11:08 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>> 
>>>>> On Sat, 6 Apr 2019, Andy Lutomirski wrote:
>>>>> On Fri, Apr 5, 2019 at 8:11 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>>>> 
>>>>> From: Andy Lutomirski <luto@...nel.org>
>>>>> 
>>>>> The IRQ stack lives in percpu space, so an IRQ handler that overflows it
>>>>> will overwrite other data structures.
>>>>> 
>>>>> 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 panic
>>>>> immediately on an IRQ stack overflow.
>>>> 
>>>> 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.)
>>>> 
>>>> 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.
>>> 
>>> 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.
>>> 
>>> 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.
>>> 
>>> 
>> Well, any code like this is already busted, since the stacks alignment
>> doesn’t really change with these patches applied.
> 
> It does. The existing code has it aligned by chance because the irq stack
> is the first entry in the per cpu area. But yes, there is no reason to require
> that alignment for irqstacks.
> 

Isn’t it the first entry in the percpu area (the normal one, not cea)?  Is that aligned beyond PAGE_SIZE?