Re: [PATCH v2 1/2] url: do not read past end of buffer

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 4, 2019, 20:27 UTC
Message-ID: <xmqq4l55t7t5.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <9628f0bfeda578a1c7d157d61b87f5c430567d74.1559670300.git.matvore@google.com>

Matthew DeVore <matvore@google.com> writes:

Show 30 quoted lines

> url_decode_internal could have been tricked into reading past the length
> of the **query buffer if there are fewer than 2 characters after a % (in
> a null-terminated string, % would have to be the last character).
> Prevent this from happening by checking len before decoding the %
> sequence.
>
> Helped-by: René Scharfe <l.s.r@web.de>
> Signed-off-by: Matthew DeVore <matvore@google.com>
> ---
>  url.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/url.c b/url.c
> index 25576c390b..9ea9d5611b 100644
> --- a/url.c
> +++ b/url.c
> @@ -39,21 +39,21 @@ static char *url_decode_internal(const char **query, int len,
>  		unsigned char c = *q;
>  
>  		if (!c)
>  			break;
>  		if (stop_at && strchr(stop_at, c)) {
>  			q++;
>  			len--;
>  			break;
>  		}
>  
> -		if (c == '%') {
> +		if (c == '%' && (len < 0 || len >= 3)) {
>  			int val = hex2chr(q + 1);

This made me wonder what happens when the caller sent -1 in len, but hex2chr() stops on such a string with % plus one hexadecimal at the end of the string, and we'd end up copying these two bytes one at a time, which is what we want, so it is OK. And the rejection of %00 done in 2/2 follows the same codeflow here, which is quite straight forward.

Nice.

Show 9 quoted lines

>  			if (0 <= val) {
>  				strbuf_addch(out, val);
>  				q += 3;
>  				len -= 3;
>  				continue;
>  			}
>  		}
>  
>  		if (decode_plus && c == '+')