Re: [PATCH] net: ipv6: Fix NULL dereference in ipv6_route_check_nh

[Sabrina Dubroca <sd@queasysnail.net>]

2025-03-26, 16:22:15 +0530, Purva Yeshi wrote:
> Fix Smatch-detected error:
> net/ipv6/route.c:3427 ip6_route_check_nh() error:
> we previously assumed '_dev' could be null

I don't think this can actually happen. ip6_route_check_nh only gets
called via fib6_nh_init -> ip6_validate_gw -> ip6_route_check_nh, and
ip6_validate_gw unconditionally does dev = *_dev. Which is fine,
because its only caller (fib6_nh_init) passes &dev, so that can't be
NULL (and same for idev).

> Ensure _dev and idev are checked for NULL before dereferencing in
> ip6_route_check_nh. Assign NULL explicitly when fib_nh_dev is NULL
> to prevent unintended dereferences.

That's a separate issue (if it's really possible - I haven't checked)
than the smatch report you're quoting above. And if it is, it would
deserve a Fixes tag for the commit introducing this code.

> 
> Signed-off-by: Purva Yeshi <purvayeshi550@gmail.com>
> ---
>  net/ipv6/route.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index ef2d23a1e3d5..ad5b3098eba0 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -3424,9 +3424,20 @@ static int ip6_route_check_nh(struct net *net,
>  		if (dev != res.nh->fib_nh_dev)
>  			err = -EHOSTUNREACH;
>  	} else {
> -		*_dev = dev = res.nh->fib_nh_dev;
> -		netdev_hold(dev, dev_tracker, GFP_ATOMIC);
> -		*idev = in6_dev_get(dev);
> +		if (res.nh->fib_nh_dev) {  /* Ensure fib_nh_dev is valid */

I don't think any of these comments are particularly helpful. It's
pretty clear that you're checking for NULL/setting NULL in all those
cases.

> +			dev = res.nh->fib_nh_dev;
> +
> +			if (_dev)  /* Only assign if _dev is not NULL */
> +				*_dev = dev;
> +
> +			netdev_hold(dev, dev_tracker, GFP_ATOMIC);
> +			*idev = in6_dev_get(dev);
> +		} else {
> +			if (_dev)
> +				*_dev = NULL;  /* Explicitly set NULL */
> +			if (idev)
> +				*idev = NULL;  /* Explicitly set NULL */
> +		}
>  	}
>  
>  	return err;

-- 
Sabrina