linux-kernel - Re: [PATCH] afs: Prefer struct_size over open coded arithmetic

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Date:   Mon, 20 Sep 2021 19:09:38 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Len Baker <len.baker@....com>, David Howells <dhowells@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-afs@...ts.infradead.org, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] afs: Prefer struct_size over open coded arithmetic



On 9/19/21 04:44, Len Baker wrote:
> As noted in the "Deprecated Interfaces, Language Features, Attributes,
> and Conventions" documentation [1], size calculations (especially
> multiplication) should not be performed in memory allocator (or similar)
> function arguments due to the risk of them overflowing. This could lead
> to values wrapping around and a smaller allocation being made than the
> caller was expecting. Using those allocations could lead to linear
> overflows of heap memory and other misbehaviors.
> 
> So, use the struct_size() helper to do the arithmetic instead of the
> argument "size + size * count" in the kzalloc() function.
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
> 
> Signed-off-by: Len Baker <len.baker@....com>

Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Thanks
--
Gustavo

> ---
>  fs/afs/security.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/afs/security.c b/fs/afs/security.c
> index 3c7a8fc4f93f..7c6a63a30394 100644
> --- a/fs/afs/security.c
> +++ b/fs/afs/security.c
> @@ -219,8 +219,7 @@ void afs_cache_permit(struct afs_vnode *vnode, struct key *key,
>  	 * yet.
>  	 */
>  	size++;
> -	new = kzalloc(sizeof(struct afs_permits) +
> -		      sizeof(struct afs_permit) * size, GFP_NOFS);
> +	new = kzalloc(struct_size(new, permits, size), GFP_NOFS);
>  	if (!new)
>  		goto out_put;
> 
> --
> 2.25.1
>