linux-hardening - Re: [PATCH] sysctl: Avoid open coded arithmetic in memory allocator functions

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Date:   Sat, 16 Oct 2021 17:18:24 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Len Baker <len.baker@....com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-hardening@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysctl: Avoid open coded arithmetic in memory allocator
 functions

On Sat, Oct 16, 2021 at 05:28:28PM +0200, Len Baker wrote:
> +static size_t new_dir_size(size_t namelen)
> +{
> +	size_t bytes;
> +
> +	if (check_add_overflow(sizeof(struct ctl_dir), sizeof(struct ctl_node),
> +			       &bytes))
> +		return SIZE_MAX;
> +	if (check_add_overflow(bytes, array_size(sizeof(struct ctl_table), 2),
> +			       &bytes))
> +		return SIZE_MAX;
> +	if (check_add_overflow(bytes, namelen, &bytes))
> +		return SIZE_MAX;
> +	if (check_add_overflow(bytes, (size_t)1, &bytes))
> +		return SIZE_MAX;
> +
> +	return bytes;
> +}

I think this is overkill.  All these structs are small and namelen is
supplied by the kernel, not specified by userspace.  It really complicates
the code, and I don't see the advantage.