linux-kernel - Re: [PATCH v3 4/4] slub: Force on no_hash_pointers when slub

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Message-ID: <202109201126.E9902480D9@keescook>
Date:   Mon, 20 Sep 2021 11:28:50 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v3 4/4] slub: Force on no_hash_pointers when slub_debug
 is enabled

On Mon, Sep 20, 2021 at 11:23:01AM -0700, Stephen Boyd wrote:
> Quoting Kees Cook (2021-09-20 07:29:54)
> > On Tue, Jun 01, 2021 at 11:22:02AM -0700, Stephen Boyd wrote:
> > > Obscuring the pointers that slub shows when debugging makes for some
> > > confusing slub debug messages:
> > >
> > >  Padding overwritten. 0x0000000079f0674a-0x000000000d4dce17
> > >
> > > Those addresses are hashed for kernel security reasons. If we're trying
> > > to be secure with slub_debug on the commandline we have some big
> > > problems given that we dump whole chunks of kernel memory to the kernel
> > > logs. Let's force on the no_hash_pointers commandline flag when
> > > slub_debug is on the commandline. This makes slub debug messages more
> > > meaningful and if by chance a kernel address is in some slub debug
> > > object dump we will have a better chance of figuring out what went
> > > wrong.
> > >
> > > Note that we don't use %px in the slub code because we want to reduce
> > > the number of places that %px is used in the kernel. This also nicely
> > > prints a big fat warning at kernel boot if slub_debug is on the
> > > commandline so that we know that this kernel shouldn't be used on
> > > production systems.
> >
> > Eeeek. I missed this patch. NAK NAK. People use slub_debug for
> > production systems to gain redzoning, etc, as a layer of defense, and
> > they absolutely do not want %p-hashing disabled. %p hashing is
> > controlled by the no_hash_pointers boot param (since v5.12), and needs to stay
> > separate from slub_debug.
> >
> > Can we please revert this in Linus's tree and in v5.14?
> >
> 
> This is fine with me as long as debugging with slub_debug on the
> commandline is possible. Would you prefer v1 of this patch series[1]
> that uses the printk format to print unhashed pointers in slub debugging
> messages?
> 
> [1] https://lore.kernel.org/r/20210520013539.3733631-1-swboyd@chromium.org

I'd like to keep %px use in the kernel minimized. Seeing full pointers (%p
hashing disabled) can be done with the no_hash_pointers boot param, and
that's used in other debug cases as well. I'd rather keep it a global
knob.

Thanks!

-Kees

-- 
Kees Cook