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Message-ID: <3e2f7e2cb4f6451a9ef5d0fb9e1f6080@AcuMS.aculab.com>
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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