Re: [PATCH -next] mm: delete oversized WARN_ON() in kvmalloc() calls

[Kees Cook]

On Thu, Dec 02, 2021 at 10:06:24AM +0800, Bixuan Cui wrote:
> Delete the WARN_ON() and return NULL directly for oversized parameter
> in kvmalloc() calls.
> Also add unlikely().
> 
> Fixes: 7661809d493b ("mm: don't allow oversized kvmalloc() calls")
> Signed-off-by: Bixuan Cui <cuibixuan@xxxxxxxxxxxxxxxxx>
> ---
> There are a lot of oversize warnings and patches about kvmalloc() calls
> recently. Maybe these warnings are not very necessary.

It seems these warnings are working, yes? i.e. we're finding the places
where giant values are coming in?

> 
> https://lore.kernel.org/all/YadOjJXMTjP85MQx@unreal
> 
> The example of size check in __do_kmalloc_node():
> __do_kmalloc_node(size_t size, gfp_t flags, int node, unsigned long caller)
> {
>         struct kmem_cache *cachep;
>         void *ret;
> 
>         if (unlikely(size > KMALLOC_MAX_CACHE_SIZE))
>                 return NULL;
>         cachep = kmalloc_slab(size, flags);
> 
>  mm/util.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 7e433690..d26f19c 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -587,7 +587,7 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
>  		return ret;
>  
>  	/* Don't even allow crazy sizes */
> -	if (WARN_ON_ONCE(size > INT_MAX))
> +	if (unlikely(size > INT_MAX))
>  		return NULL;

If we're rejecting the value, then it's still a pathological size, so
shouldn't the check be happening in the caller? I think the WARN is
doing exactly what it was supposed to do: find the places where bad
sizes can reach vmalloc.

-Kees

>  
>  	return __vmalloc_node(size, 1, flags, node,
> -- 
> 1.8.3.1
> 

-- 
Kees Cook