[PATCH v4] of: fdt: fix possible overflow during parsing of fdt

[Ahmad Fatoum]

On 14.11.24 16:51, Abdelrahman Youssef wrote:
> While fuzzing, the name marked by FDT_BEGIN_NODE sometimes extends beyond
> the struct block area, causing a heap-overflow.
> 
> Since `maxlen` is an unsigned integer representing the length of name,
> It can be negative, so it overflows to large numbers, Causing strnlen()
> to overflow.
> 
> So we can just change the type of maxlen to signed and check if it's a
> non-positive value, because name has a minimum length of 1 byte ('\0').
> 
> Also in strnlen() we shouldn't check for bytes exceeding maxlen, so we can remove
> + 1 in strnlen(). We also change if (len > maxlen) to >= to count for the null
> terminator.
> 
> Signed-off-by: Abdelrahman Youssef <abdelrahmanyossef12 at gmail.com>

Reviewed-by: Ahmad Fatoum <a.fatoum at pengutronix.de>

> 
> ---
> v3 -> v4:
>   - replace maxlen < 0 to maxlen <= 0 (Sascha)
>   - remove + 1 in strnlen() (Sascha)
> v2 -> v3
>   - changed formatting
> v1 -> v2
>   - the overflow was due to integer overflow not out-of-bounds (Ahmad)
> ---
>  drivers/of/fdt.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index 2c3ea31394..75af1844f3 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -176,7 +176,7 @@ static struct device_node *__of_unflatten_dtb(const void *infdt, int size,
>  	void *dt_strings;
>  	struct fdt_header f;
>  	int ret;
> -	unsigned int maxlen;
> +	int maxlen;
>  	const struct fdt_header *fdt = infdt;
>  
>  	ret = fdt_parse_header(infdt, size, &f);
> @@ -210,8 +210,13 @@ static struct device_node *__of_unflatten_dtb(const void *infdt, int size,
>  			maxlen = (unsigned long)fdt + f.off_dt_struct +
>  				f.size_dt_struct - (unsigned long)name;
>  
> -			len = strnlen(name, maxlen + 1);
> -			if (len > maxlen) {
> +			if (maxlen <= 0) {
> +				ret = -ESPIPE;
> +				goto err;
> +			}
> +
> +			len = strnlen(name, maxlen);
> +			if (len >= maxlen) {
>  				ret = -ESPIPE;
>  				goto err;
>  			}


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