linux-kernel - Re: [PATCH 11/21] x86/dumpstack: Handle stack overflow on all stacks

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Message-ID: <CALCETrXptow2p4N4zqCny-inHyEcUw4hemDnvw50i5BcKx0G6w@mail.gmail.com>
Date:   Mon, 27 Nov 2017 21:29:26 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 11/21] x86/dumpstack: Handle stack overflow on all stacks

On Mon, Nov 27, 2017 at 8:29 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Mon, Nov 27, 2017 at 11:26:30AM -0800, Linus Torvalds wrote:
>> On Mon, Nov 27, 2017 at 2:45 AM, Ingo Molnar <mingo@...nel.org> wrote:
>> > From: Andy Lutomirski <luto@...nel.org>
>> >
>> > We currently special-case stack overflow on the task stack.  We're
>> > going to start putting special stacks in the fixmap with a custom
>> > layout, so they'll have guard pages, too.  Teach the unwinder to be
>> > able to unwind an overflow of any of the stacks.
>>
>> Why isn't this together with 01/21? The two cases seem to be entirely
>> identical and fundamentally the same issue.
>
> Yeah, they probably do belong in the same patch.
>
>> In fact, maybe the whole "stack overflow" special cases should be in
>> "get_stack_info()" itself, rather than be special-cased in the
>> callers?
>
> I would be nervous about doing that.  Several of the get_stack_info()
> callers rely on it being honest.
>
> In fact, looking deeper at the above patch, it doesn't seem convincingly
> safe to me.  What if the adjacent page doesn't exist?  Then when the
> oops dumping code dereferences the 'stack' variable, you get an oops in
> your oops.
>

Isn't the oops dumping code supposed to dereference everything using a
special safe function?

Anyway, get_stack_info() wouldn't really be lying.  It would just be
returning something where begin..end doesn't contain the requested
pointer.