linux-kernel - Re: [RFC PATCH 1/2] x86: WARN() when uaccess helpers fault on kernel addresses

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Message-Id: <80A71AAC-89B0-448A-8FF6-FFF1B68270A2@amacapital.net>
Date:   Tue, 7 Aug 2018 07:02:10 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kernel-hardening@...ts.openwall.com,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH 1/2] x86: WARN() when uaccess helpers fault on kernel addresses


> On Aug 7, 2018, at 4:04 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> 
>> On Tue, Aug 7, 2018 at 3:22 AM, Jann Horn <jannh@...gle.com> wrote:
>> There have been multiple kernel vulnerabilities that permitted userspace to
>> pass completely unchecked pointers through to userspace accessors:
>> 
>> - the waitid() bug - commit 96ca579a1ecc ("waitid(): Add missing
>>   access_ok() checks")
>> - the sg/bsg read/write APIs
>> - the infiniband read/write APIs
>> 
>> These don't happen all that often, but when they do happen, it is hard to
>> test for them properly; and it is probably also hard to discover them with
>> fuzzing. Even when an unmapped kernel address is supplied to such buggy
>> code, it just returns -EFAULT instead of doing a proper BUG() or at least
>> WARN().
>> 
>> This patch attempts to make such misbehaving code a bit more visible by
>> WARN()ing in the pagefault handler code when a userspace accessor causes
>> #PF on a kernel address and the current context isn't whitelisted.
> 
> This is not triggerable unless there is a kernel bug, right? I mean
> this won't be a DoS vector? And any case is something to report to
> kernel developers?

Yes. I expect it to help fuzzers, since it will make a uaccess at a bad address much more likely to oops.

My old series found one bug when the automated fuzzers fuzzed it. That bug is fixed now.