Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   David Laight <David.Laight@ACULAB.COM>
To:     'Jann Horn' <jannh@google.com>, Kees Cook <keescook@chromium.org>,
        "linux-hardening@vger.kernel.org" <linux-hardening@vger.kernel.org>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>
CC:     Greg KH <gregkh@linuxfoundation.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        Seth Jenkins <sethjenkins@google.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Andy Lutomirski <luto@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH] exit: Put an upper limit on how often we can oops
Thread-Topic: [PATCH] exit: Put an upper limit on how often we can oops
Thread-Index: AQHY8uVtJeqm4QnZJEegjt8nfpQUG640wW1Q
Date:   Tue, 8 Nov 2022 09:26:06 +0000
Message-ID: <3e2f7e2cb4f6451a9ef5d0fb9e1f6080@AcuMS.aculab.com>
References: <20221107201317.324457-1-jannh@google.com>
In-Reply-To: <20221107201317.324457-1-jannh@google.com>
Accept-Language: en-GB, en-US
MIME-Version: 1.0
Content-Language: en-US
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Precedence: bulk

From: Jann Horn
> Sent: 07 November 2022 20:13
> 
> Many Linux systems are configured to not panic on oops; but allowing an
> attacker to oops the system **really** often can make even bugs that look
> completely unexploitable exploitable (like NULL dereferences and such) if
> each crash elevates a refcount by one or a lock is taken in read mode, and
> this causes a counter to eventually overflow.
> 
> The most interesting counters for this are 32 bits wide (like open-coded
> refcounts that don't use refcount_t). (The ldsem reader count on 32-bit
> platforms is just 16 bits, but probably nobody cares about 32-bit platforms
> that much nowadays.)
> 
> So let's panic the system if the kernel is constantly oopsing.

I think you are pretty much guaranteed to run out of memory
(or at least KVA) before any 32bit counter wraps.

That is probably even harder to diagnose than a refcount wrap!

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)