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