Date: Wed, 9 Nov 2022 09:04:36 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Jann Horn' <jannh@...gle.com> CC: 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>, 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: 08 November 2022 14:53 > > On Tue, Nov 8, 2022 at 10:26 AM David Laight <David.Laight@...lab.com> wrote: > > > 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. > > Not if you repeatedly take a reference and then oops without dropping > the reference, and the oops path cleans up all the resources that were > allocated for the crashing tasks. In that case, each oops increments > the reference count by 1 without causing memory allocation. I'd have thought that the kernel stack and process areas couldn't be freed because they might contain 'live data'. There is also the much smaller pid_t structure. Of course I might be wrong... But I'm sure /proc/pid/stack is valid for an oopsed process. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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