Re: [PATCH v4 3/3] keys, dns: Allow key types (eg. DNS) to be reclaimed immediately on expiry

[Simon Horman]

+ Edward Adam Davis

On Thu, Dec 21, 2023 at 01:45:30PM +0000, David Howells wrote:
> If a key has an expiration time, then when that time passes, the key is
> left around for a certain amount of time before being collected (5 mins by
> default) so that EKEYEXPIRED can be returned instead of ENOKEY.  This is a
> problem for DNS keys because we want to redo the DNS lookup immediately at
> that point.
> 
> Fix this by allowing key types to be marked such that keys of that type
> don't have this extra period, but are reclaimed as soon as they expire and
> turn this on for dns_resolver-type keys.  To make this easier to handle,
> key->expiry is changed to be permanent if TIME64_MAX rather than 0.
> 
> Furthermore, give such new-style negative DNS results a 1s default expiry
> if no other expiry time is set rather than allowing it to stick around
> indefinitely.  This shouldn't be zero as ls will follow a failing stat call
> immediately with a second with AT_SYMLINK_NOFOLLOW added.
> 
> Fixes: 1a4240f4764a ("DNS: Separate out CIFS DNS Resolver code")
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> Tested-by: Markus Suvanto <markus.suvanto@xxxxxxxxx>

...

> diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
> index 01e54b46ae0b..2a6d363763a2 100644
> --- a/net/dns_resolver/dns_key.c
> +++ b/net/dns_resolver/dns_key.c
> @@ -91,6 +91,7 @@ const struct cred *dns_resolver_cache;
>  static int
>  dns_resolver_preparse(struct key_preparsed_payload *prep)
>  {
> +	const struct dns_server_list_v1_header *v1;
>  	const struct dns_payload_header *bin;
>  	struct user_key_payload *upayload;
>  	unsigned long derrno;
> @@ -122,6 +123,13 @@ dns_resolver_preparse(struct key_preparsed_payload *prep)
>  			return -EINVAL;
>  		}
>  
> +		v1 = (const struct dns_server_list_v1_header *)bin;
> +		if ((v1->status != DNS_LOOKUP_GOOD &&
> +		     v1->status != DNS_LOOKUP_GOOD_WITH_BAD)) {
> +			if (prep->expiry == TIME64_MAX)
> +				prep->expiry = ktime_get_real_seconds() + 1;
> +		}
> +
>  		result_len = datalen;
>  		goto store_result;
>  	}

Hi David,

As has been pointed out by Edward Adam Davis, this may result
in a buffer overrun. Just above this hunk the following length
check occurs:

		if (datalen <= sizeof(*bin))
			return -EINVAL;

But the new code above reads beyond the end of sizeof(*bin).

Link: https://lore.kernel.org/netdev/tencent_7D663C8936BA96F837124A4474AF76ED6709@xxxxxx/

...