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

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Message-ID: <CA+55aFwK1asVEOwLSji4K7bT=uidxmDVq-r+_6YUbBnxCOkxOQ@mail.gmail.com>
Date:   Tue, 23 Aug 2016 14:47:36 -0400
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        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>,
        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 9:27 PM, Kees Cook <keescook@...omium.org> wrote:
>
> I need to re-check the copy_*_user changes, but on several
> architectures, the bounds checking is only triggered for non
> built-in-const sizes, so these kinds of pointless checks shouldn't
> happen.

They definitely happen at least on x86.

"stat()" is one common user of fixed-sized structures being copied.
There are tons of others, but 'stat()' is the one I've seen in my
profiles before as being noticeable. It's been critical enough that I
have occasionally tried to play with making it avoid the "copy to
temporary struct, then copy_to_user() the whole struct" and just do it
field-by-field. But it gets nasty with the padding fields etc, so it's
never been done.

Not doing the access size checks for constant-sized copies (at least
when they are "sufficiently small" constants) would probably be the
right thing to do, and then depend on gcc just getting the static case
right warning-wise. Which isn't apparently getting done right now
either, but oh well..

            Linus