Re: [PATCH v2 6/9] gpg-interface: do not hardcode the key string len anymore

From: Jeff King <peff@peff.net>
Date: Jul 10, 2018, 15:49 UTC
Message-ID: <20180710154931.GA23624@sigill.intra.peff.net>
In-Reply-To: <7300c85a1c6d484c781cc80b307b5e7f085ac226.1531208187.git.henning.schild@siemens.com>

On Tue, Jul 10, 2018 at 10:52:28AM +0200, Henning Schild wrote:

> gnupg does print the keyid followed by a space and the signer comes
> next. The same pattern is also used in gpgsm, but there the key length
> would be 40 instead of 16. Instead of hardcoding the expected length,
> find the first space and calculate it.

Sounds good, but I think there's an off-by-one in the patch.

Show 11 quoted lines

> diff --git a/gpg-interface.c b/gpg-interface.c
> index 0a8d1bff3..ac2df498d 100644
> --- a/gpg-interface.c
> +++ b/gpg-interface.c
> @@ -88,10 +88,11 @@ static void parse_gpg_output(struct signature_check *sigc)
>  		sigc->result = sigcheck_gpg_status[i].result;
>  		/* The trust messages are not followed by key/signer information */
>  		if (sigc->result != 'U') {
> -			sigc->key = xmemdupz(found, 16);
> +			next = strchrnul(found, ' ');
> +			sigc->key = xmemdupz(found, next - found);

Here "next" may point to the trailing NUL of the string...

Show 5 quoted lines

>  			/* The ERRSIG message is not followed by signer information */
>  			if (sigc-> result != 'E') {
> -				found += 17;
> +				found = next + 1;
>  				next = strchrnul(found, '\n');

...in which case "found" points past the end of the string, and we search random memory. That's presumably impossible with well-formed gpg output (you don't get 'E' without an extra message), but we should be robust against bogus input.

In the general case you need:

  found = *next ? next + 1 : next;

or similar. In this case, you can actually do:

  found = next;

because we know that it's OK to search over the literal space again. But that's pretty subtle, so we're probably better off just doing the conditional above.

(And yes, looking at the existing code, I think it's even worse, as there does not seem to be a guarantee that we even have 16 characters in the string).

-Peff