linux-kernel - Re: [PATCH v4 54/57] x86/mm: convert arch_within_stack

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Date:   Tue, 23 Aug 2016 11:06:29 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH v4 54/57] x86/mm: convert arch_within_stack_frames() to
 use the new unwinder

On Mon, Aug 22, 2016 at 03:11:32PM -0700, Linus Torvalds wrote:
> On Thu, Aug 18, 2016 at 6:06 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > Convert arch_within_stack_frames() to use the new unwinder.
> 
> Please don't do this.
> 
> There's no real reason to unwind the stack frame. If it's not on the
> current stack page, it shouldn't be a valid source anyway, so
> unwidning things just seems entirely pointless.
> 
> Quite frankly, I think the whole "look at the stack frames" logic
> should be removed from this. It's classic crap that external patches
> do. How many call-sites does it actually check, and how many of them
> aren't already checked by the existing static checks for constant
> addresses within existing objects?

I noticed the __compiletime_object_size() check is completely disabled
for gcc >= 4.6, thanks to:

  2fb0815c9ee6 ("gcc4: disable __compiletime_object_size for GCC 4.6+")

AFAICT, that change went too far: it disabled both the compile-time
*and* the runtime checks, so copy_from_user_overflow() is never called.

Working on a couple of patches to try to fix that.

-- 
Josh