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

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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

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