On Mon, 29 Jul 2024 15:49:00 +0200
Lukas Wunner <lukas@xxxxxxxxx> 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@xxxxxxxxx>
Change looks fine to me.
Random musing inline.
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> ---
> 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);
>
