Re: [PATCH 00/10] Hardening page _refcount

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Wed, Dec 8, 2021 at 4:05 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Dec 08, 2021 at 08:35:34PM +0000, Pasha Tatashin wrote:
> > It is hard to root cause _refcount problems, because they usually
> > manifest after the damage has occurred.  Yet, they can lead to
> > catastrophic failures such memory corruptions. There were a number
> > of refcount related issues discovered recently [1], [2], [3].
> >
> > Improve debugability by adding more checks that ensure that
> > page->_refcount never turns negative (i.e. double free does not
> > happen, or free after freeze etc).
> >
> > - Check for overflow and underflow right from the functions that
> >   modify _refcount
> > - Remove set_page_count(), so we do not unconditionally overwrite
> >   _refcount with an unrestrained value
> > - Trace return values in all functions that modify _refcount
>

Hi Matthew,

Thank you for looking at this series.

> You're doing a lot more atomic instructions with these patches.

This is not exactly so. There are no *more* atomic instructions. There
are, however, different atomic instructions:

For example:  atomic_add() becomes atomic_fetch_add()

On x86 it is:

atomic_add:
    lock add %eax,(%rsi)

atomic_fetch_add:
    lock xadd %eax,(%rsi)

On ARM64, I believe the same CAS instruction is used for both.

  Have you
> done any performance measurements with these patches applied and debug
> disabled?

Yes, I have done some performance tests exactly as you described with
CONFIG_DEBUG_VM disabled and these patches applied.
I tried: hackbench, unixbench, and a few more benchmarks; I did not
see any performance difference.

>  I'm really not convinced it's worth closing
> one-instruction-wide races of this kind when they are "shouldn't ever
> happen" situations.  If the debugging will catch the problem in 99.99%
> of cases and miss 0.01% without using atomic instructions, that seems
> like a better set of tradeoffs than catching 100% of problems by using
> the atomic instructions.

I think we should relax the precise catching of bugs only if there is
indeed a measurable performance impact. The problem is that if there
is a __refcount bug, the security consequences are dire as it may lead
to leaking memory from one process to another.

Thanks,
Pasha