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

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
Date: Tue, 8 Nov 2022 09:26:06 +0000	[thread overview]
Message-ID: <3e2f7e2cb4f6451a9ef5d0fb9e1f6080@AcuMS.aculab.com> (raw)
In-Reply-To: <20221107201317.324457-1-jannh@google.com>

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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