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Date: Tue, 10 Oct 2023 16:09:35 -0700
From: Kees Cook <keescook@...omium.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Russell King <linux@...linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] atags_proc: Add __counted_by for struct buffer and
 use struct_size()

On Tue, Oct 10, 2023 at 06:46:50AM -0600, Gustavo A. R. Silva wrote:
> Prepare for the coming implementation by GCC and Clang of the __counted_by
> attribute. Flexible array members annotated with __counted_by can have
> their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
> array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
> functions).
> 
> While there, use struct_size() helper, instead of the open-coded
> version, to calculate the size for the allocation of the whole
> flexible structure, including of course, the flexible-array member.
> 
> This code was found with the help of Coccinelle, and audited and
> fixed manually.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  arch/arm/kernel/atags_proc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/kernel/atags_proc.c b/arch/arm/kernel/atags_proc.c
> index 3ec2afe78423..cd09f8ab93e3 100644
> --- a/arch/arm/kernel/atags_proc.c
> +++ b/arch/arm/kernel/atags_proc.c
> @@ -7,7 +7,7 @@
>  
>  struct buffer {
>  	size_t size;
> -	char data[];
> +	char data[] __counted_by(size);
>  };
>  
>  static ssize_t atags_read(struct file *file, char __user *buf,
> @@ -54,7 +54,7 @@ static int __init init_atags_procfs(void)
>  
>  	WARN_ON(tag->hdr.tag != ATAG_NONE);
>  
> -	b = kmalloc(sizeof(*b) + size, GFP_KERNEL);
> +	b = kmalloc(struct_size(b, data, size), GFP_KERNEL);
>  	if (!b)
>  		goto nomem;
>  

Yup, "size" is assigned before accessing "data".

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

-- 
Kees Cook

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