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

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Message-Id: <50F7A079-11F5-45EA-B378-7527B5920A7C@amacapital.net>
Date:   Sun, 7 Apr 2019 02:28:07 -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 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.