linux-hardening - Re: [PATCH 1/2] mm/slub: remove redundant kasan_reset_tag() from freelist

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Message-ID: <202307110914.8D460C7@keescook>
Date:   Tue, 11 Jul 2023 09:14:49 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Matteo Rizzo <matteorizzo@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev@...glegroups.com, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/slub: remove redundant kasan_reset_tag() from
 freelist_ptr calculations

On Tue, Jul 11, 2023 at 03:46:24PM +0200, Vlastimil Babka wrote:
> Commit d36a63a943e3 ("kasan, slub: fix more conflicts with
> CONFIG_SLAB_FREELIST_HARDENED") has introduced kasan_reset_tags() to
> freelist_ptr() encoding/decoding when CONFIG_SLAB_FREELIST_HARDENED is
> enabled to resolve issues when passing tagged or untagged pointers
> inconsistently would lead to incorrect calculations.
> 
> Later, commit aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing
> metadata") made sure all pointers have tags reset regardless of
> CONFIG_SLAB_FREELIST_HARDENED, because there was no other way to access
> the freepointer metadata safely with hw tag-based KASAN.
> 
> Therefore the kasan_reset_tag() usage in freelist_ptr_encode()/decode()
> is now redundant, as all callers use kasan_reset_tag() unconditionally
> when constructing ptr_addr. Remove the redundant calls and simplify the
> code and remove obsolete comments.
> 
> Also in freelist_ptr_encode() introduce an 'encoded' variable to make
> the lines shorter and make it similar to the _decode() one.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Thanks, this is much more readable!

Acked-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook