linux-kernel - Re: [RFC PATCH 01/14] mm/slub: don't try to dereference invalid freepointers

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Date:   Fri, 15 Sep 2023 13:50:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Matteo Rizzo <matteorizzo@...gle.com>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org, vbabka@...e.cz,
        roman.gushchin@...ux.dev, 42.hyeyoo@...il.com,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-hardening@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        corbet@....net, luto@...nel.org, peterz@...radead.org,
        jannh@...gle.com, evn@...gle.com, poprdi@...gle.com,
        jordyzomer@...gle.com
Subject: Re: [RFC PATCH 01/14] mm/slub: don't try to dereference invalid
 freepointers

On Fri, Sep 15, 2023 at 10:59:20AM +0000, Matteo Rizzo wrote:
> slab_free_freelist_hook tries to read a freelist pointer from the
> current object even when freeing a single object. This is invalid
> because single objects don't actually contain a freelist pointer when
> they're freed and the memory contains other data. This causes problems
> for checking the integrity of freelist in get_freepointer.
> 
> Signed-off-by: Matteo Rizzo <matteorizzo@...gle.com>
> ---
>  mm/slub.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f7940048138c..a7dae207c2d2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1820,7 +1820,9 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
>  
>  	do {
>  		object = next;
> -		next = get_freepointer(s, object);
> +		/* Single objects don't actually contain a freepointer */
> +		if (object != old_tail)
> +			next = get_freepointer(s, object);
>  
>  		/* If object's reuse doesn't have to be delayed */
>  		if (!slab_free_hook(s, object, slab_want_init_on_free(s))) {

I find this loop's logic a bit weird, but, yes, this ends up being
correct and avoids needless work.

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

-- 
Kees Cook