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Date:   Thu, 15 Oct 2020 15:03:43 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Christopher Lameter <cl@linux.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Waiman Long <longman@redhat.com>,
        Marco Elver <elver@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Roman Gushchin <guro@fb.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v3 0/3] Actually fix freelist pointer vs redzoning
Message-ID: <202010151501.C9F9D2ACF@keescook>
References: <20201015033712.1491731-1-keescook@chromium.org>
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On Thu, Oct 15, 2020 at 11:44:15AM +0200, Vlastimil Babka wrote:
> On 10/15/20 10:23 AM, Christopher Lameter wrote:
> > On Wed, 14 Oct 2020, Kees Cook wrote:
> > 
> > > Note on patch 2: Christopher NAKed it, but I actually think this is a
> > > reasonable thing to add -- the "too small" check is only made when built
> > > with CONFIG_DEBUG_VM, so it *is* actually possible for someone to trip
> > > over this directly, even if it would never make it into a released
> > > kernel. I see no reason to just leave this foot-gun in place, though, so
> > > we might as well just fix it too. (Which seems to be what Longman was
> > > similarly supporting, IIUC.)
> > 
> > Well then remove the duplication of checks. The NAK was there because it
> > seems that you were not aware of the existing checks.
> > 
> > > Anyway, if patch 2 stays NAKed, that's fine. It's entirely separable,
> > > and the other 2 can land. :)
> > 
> > Just deal with the old checks too and it will be fine.
> 
> Yeah, the existing check is under CONFIG_DEBUG_VM, which means it's not
> active on some configurations. Creating a cache is not exactly fast path
> operation, so I would remove this guard.
> As for the minimum size check, I would probably remove it (but watch out if
> SLAB/SLOB can handle it). It's not effective to use slab cache for 4-byte
> objects, but why make it an error.

Err, why did the check exist to begin with? If the check isn't wanted,
that's one thing, but I was just trying to fix what I saw in the redzone
handling. What is preferred here?

1) drop patch 2
2) keep patch 2, but also:
	a) validate slab/slob can handle < word-sized allocations
	b) remove check in kmem_cache_sanity_check

option 2a seems like it could be fragile if I miss something. I think
I'd rather just take option 1.

-- 
Kees Cook