patch, 2 partsurl: do not allow %00 to represent NULL in URLs

> There is no reason to allow %00 to terminate a string, so do not allow it.
> Otherwise, we end up returning arbitrary content in the string (that which is
> after the %00) which is effectively hidden from callers and can escape sanity
> checks and validation, and possible be used in tandem with a security
> vulnerability to introduce a payload.

> It looks like several of the places we do this are in the credential
> manager code, and I think I can agree that usernames and passwords
> should not contain NUL characters (for Basic auth, RFC 7617 prohibits
> it). It also seems that the credential code decodes the path parameter
> before passing it on, which is unfortunate, but can't be changed for
> backward compatibility reasons.
> 
> And then the other instances are a file: URL in remote-testsvn.c and
> query parameters that have no reason to contain NULs in http-backend.c.

> There is no reason to allow %00 to terminate a string, so do not allow it.
> Otherwise, we end up returning arbitrary content in the string (that which is
> after the %00) which is effectively hidden from callers and can escape sanity
> checks and validation, and possible be used in tandem with a security
> vulnerability to introduce a payload.

>
> 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 c0bb4e23c3..cf791cb139 100644
> --- a/url.c
> +++ b/url.c
> @@ -41,21 +41,21 @@ static char *url_decode_internal(const char **query, int len,
>  		if (!c)
>  			break;
>  		if (stop_at && strchr(stop_at, c)) {
>  			q++;
>  			len--;
>  			break;
>  		}
>
>  		if (c == '%' && len >= 3) {
>  			int val = hex2chr(q + 1);
> -			if (0 <= val) {
> +			if (0 < val) {
>  				strbuf_addch(out, val);
>  				q += 3;
>  				len -= 3;
>  				continue;
>  			}
>  		}
>
>  		if (decode_plus && c == '+')
>  			strbuf_addch(out, ' ');
>  		else
>

> It's a bit hard to see with the (extended, but still) limited context,
> but url_decode_internal() effectively returns a NUL-terminated string,
> even though it does use a strbuf parameter named "out" for temporary
> storage.  So callers really have no use for decoded NULs, and this
> change thus makes sense to me.
> 

> 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.
>
> 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..c0bb4e23c3 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 >= 3) {

>  			int val = hex2chr(q + 1);
>  			if (0 <= val) {
>  				strbuf_addch(out, val);
>  				q += 3;
>  				len -= 3;
>  				continue;
>  			}
>  		}
>
>  		if (decode_plus && c == '+')
>

> Am 03.06.19 um 22:45 schrieb Matthew DeVore:
> > 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.
> >
> > 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..c0bb4e23c3 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 >= 3) {
> 
> Tricky.  hex2chr() makes sure to not run over the end of NUL-terminated
> strings, but url_decode_internal() is supposed to honor the parameter
> len as well.  Your change disables %-decoding for the two callers that
> pass -1 as len, though.  So perhaps like this?
> 
> 		if (c == '%' && (len < 0 || len >= 3)) {