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Date: Tue, 8 Nov 2022 09:26:06 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Jann Horn' <>, Kees Cook <>,
	"" <>,
	"" <>
CC: Greg KH <>, Linus Torvalds
	<>, Seth Jenkins <>, "Eric
 W . Biederman" <>, Andy Lutomirski <>,
	"" <>
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!


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