netdev - Re: [PATCH iproute2] ipaddress: strengthen check on 'label' input

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Date:   Thu, 26 Apr 2018 14:45:39 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Patrick Talbert <ptalbert@...hat.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH iproute2] ipaddress: strengthen check on 'label' input

On Tue, 24 Apr 2018 16:08:21 +0200
Patrick Talbert <ptalbert@...hat.com> wrote:

> As mentioned in the ip-address man page, an address label must
> be equal to the device name or prefixed by the device name
> followed by a colon. Currently the only check on this input is
> to see if the device name appears at the beginning of the label
> string.
> 
> This commit adds an additional check to ensure label == dev or
> continues with a colon.
> 
> Signed-off-by: Patrick Talbert <ptalbert@...hat.com>
> ---
>  ip/ipaddress.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/ip/ipaddress.c b/ip/ipaddress.c
> index aecc9a1..edcf821 100644
> --- a/ip/ipaddress.c
> +++ b/ip/ipaddress.c
> @@ -2168,9 +2168,14 @@ static int ipaddr_modify(int cmd, int flags, int argc, char **argv)
>  		fprintf(stderr, "Not enough information: \"dev\" argument is required.\n");
>  		return -1;
>  	}
> -	if (l && matches(d, l) != 0) {
> -		fprintf(stderr, "\"dev\" (%s) must match \"label\" (%s).\n", d, l);
> -		return -1;
> +	if (l) {
> +		size_t d_len = strlen(d);
> +
> +		if (!(matches(d, l) == 0 && (l[d_len] == '\0' || l[d_len] == ':'))) {

matches is not what you want here. matches does prefix match (ie matches("eth0", "eth") == 0).
Also, what if label is shorter than the device, you would end up dereferencing past
the end of the string!

I think you want something like:

static bool is_valid_label(const char *dev, const char *label)
{
	const char *sep;

	sep = strchr(label, ':');
	if (sep)
		return strncmp(dev, label, sep - label) == 0;
	else
		return strcmp(dev, label) == 0;

}

> +			fprintf(stderr, "\"label\" (%s) must match \"dev\" (%s) or be prefixed by"
> +				" \"dev\" with a colon.\n", l, d);
> +			return -1;
> +		}
>  	}
>  
>  	if (peer_len == 0 && local_len) {