Re: [PATCH 3/5] crypto: ecdsa - Avoid signed integer overflow on signature decoding

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Mon, 29 Jul 2024 15:49:00 +0200
Lukas Wunner <lukas@wunner.de> wrote:

> When extracting a signature component r or s from an ASN.1-encoded
> integer, ecdsa_get_signature_rs() subtracts the expected length
> "bufsize" from the ASN.1 length "vlen" (both of unsigned type size_t)
> and stores the result in "diff" (of signed type ssize_t).
> 
> This results in a signed integer overflow if vlen > SSIZE_MAX + bufsize.
> 
> The kernel is compiled with -fno-strict-overflow, which implies -fwrapv,
> meaning signed integer overflow is not undefined behavior.  And the
> function does check for overflow:
> 
>        if (-diff >= bufsize)
>                return -EINVAL;
> 
> So the code is fine in principle but not very obvious.  In the future it
> might trigger a false-positive with CONFIG_UBSAN_SIGNED_WRAP=y.
> 
> Avoid by comparing the two unsigned variables directly and erroring out
> if "vlen" is too large.
> 
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
Change looks fine to me.
Random musing inline.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

> ---
>  crypto/ecdsa.c | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
> index f63731fb7535..03f608132242 100644
> --- a/crypto/ecdsa.c
> +++ b/crypto/ecdsa.c
> @@ -35,29 +35,20 @@ static int ecdsa_get_signature_rs(u64 *dest, size_t hdrlen, unsigned char tag,
>  				  const void *value, size_t vlen, unsigned int ndigits)
>  {
>  	size_t bufsize = ndigits * sizeof(u64);
> -	ssize_t diff = vlen - bufsize;
>  	const char *d = value;
>  
> -	if (!value || !vlen)
> +	if (!value || !vlen || vlen > bufsize + 1)

Given vlen and bufsize unsigned. Even in the weird case of bufsize + 1 == 0
vlen cannot be zero.  So could drop the second condition? (or am I
missing something?) Maybe it's easier to reason that vlen == 0 is invalid though.


>  		return -EINVAL;
>  
> -	/* diff = 0: 'value' has exacly the right size
> -	 * diff > 0: 'value' has too many bytes; one leading zero is allowed that
> -	 *           makes the value a positive integer; error on more
> -	 * diff < 0: 'value' is missing leading zeros
> -	 */
> -	if (diff > 0) {
> +	if (vlen > bufsize) {
>  		/* skip over leading zeros that make 'value' a positive int */
>  		if (*d == 0) {
>  			vlen -= 1;
> -			diff--;
>  			d++;
> -		}
> -		if (diff)
> +		} else {
>  			return -EINVAL;
> +		}
>  	}
> -	if (-diff >= bufsize)
> -		return -EINVAL;
>  
>  	ecc_digits_from_bytes(d, vlen, dest, ndigits);
>