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)