linux-kernel - Re: [PATCH 1/3] big key: get rid of stack array allocation

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Message-ID: <CAGXu5j+SXCJpm9xnX1UaY0Qz2+E0qNETc+s8kwrH6hE9LLjgpw@mail.gmail.com>
Date:   Tue, 24 Apr 2018 13:04:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Tycho Andersen <tycho@...ho.ws>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Eric Biggers <ebiggers3@...il.com>,
        David Howells <dhowells@...hat.com>, keyrings@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        James Morris <jmorris@...ei.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH 1/3] big key: get rid of stack array allocation

On Tue, Apr 24, 2018 at 12:58 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> Quoting Tycho Andersen (tycho@...ho.ws):
>> On Tue, Apr 24, 2018 at 11:46:38PM +0900, Tetsuo Handa wrote:
>> > Tycho Andersen wrote:
>> > > > > +     if (unlikely(crypto_aead_ivsize(big_key_aead) != GCM_AES_IV_SIZE)) {
>> > > > > +             WARN(1, "big key algorithm changed?");
>> >
>> > Please avoid using WARN() WARN_ON() etc.
>> > syzbot would catch it and panic() due to panic_on_warn == 1.
>>
>> But it is really a programming bug in this case (and it seems better
>> than BUG()...). Isn't this exactly the sort of case we want to catch?
>>
>> Tycho
>
> Right - is there a url to some discussion about this?  Because not
> using WARN when WARN should be used, because it troubles a bot, seems
> the wrong solution.  If this *is* what's been agreed upon, then
> what is the new recommended thing to do here?

BUG() is basically supposed to never be used, as decreed by Linus.
WARN() here is entirely correct: if we encounter a case where
crypto_aead_ivsize(big_key_aead) != GCM_AES_IV_SIZE is not true, we
run the risk of stack memory corruption. If this is an EXPECTED
failure case, then okay, drop the WARN() but we have to keep the
-EINVAL.

-Kees

-- 
Kees Cook
Pixel Security