linux-hardening - RE: [PATCH] exit: Put an upper limit on how often we can oops

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Date:   Tue, 8 Nov 2022 09:26:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jann Horn' <jannh@...gle.com>, Kees Cook <keescook@...omium.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
CC:     Greg KH <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Seth Jenkins <sethjenkins@...gle.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] exit: Put an upper limit on how often we can oops

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

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