Re: [PATCH nf] netfilter: nft_set_pipapo: do not free live element

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 10 Apr 2024 16:48:49 +0200
Florian Westphal <fw@strlen.de> wrote:

> Pablo reports a crash with large batches of elements with a
> back-to-back add/remove pattern.  Quoting Pablo:
> 
>   add_elem("00000000") timeout 100 ms
>   ...
>   add_elem("0000000X") timeout 100 ms
>   del_elem("0000000X") <---------------- delete one that was just added
>   ...
>   add_elem("00005000") timeout 100 ms
> 
>   1) nft_pipapo_remove() removes element 0000000X
>   Then, KASAN shows a splat.
> 
> Looking at the remove function there is a chance that we will drop a
> rule that maps to a non-deactivated element.
> 
> Removal happens in two steps, first we do a lookup for key k and return the
> to-be-removed element and mark it as inactive in the next generation.
> Then, in a second step, the element gets removed from the set/map.
> 
> The _remove function does not work correctly if we have more than one
> element that share the same key.
> 
> This can happen if we insert an element into a set when the set already
> holds an element with same key, but the element mapping to the existing
> key has timed out or is not active in the next generation.

Uh-oh, I didn't imagine that could happen, thanks for fixing this.

The fix looks correct to me. Just one nit:

> In such case its possible that removal will unmap the wrong element.
> If this happens, we will leak the non-deactivated element, it becomes
> unreachable.
> 
> The element that got deactivated (and will be freed later) will
> remain reachable in the set data structure, this can result in
> a crash when such an element is retrieved during lookup (stale
> pointer).
> 
> Add a check that the fully matching key does in fact map to the element
> that we have marked as inactive in the deactivation step.
> If not, we need to continue searching.
> 
> Add a bug/warn trap at the end of the function as well, the remove
> function must not ever be called with an invisible/unreachable/non-existent
> element.
> 
> Fixes: 3c4287f62044 ("nf_tables: Add set type for arbitrary concatenation of ranges")
> Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
> Signed-off-by: Florian Westphal <fw@strlen.de>
> ---
>  net/netfilter/nft_set_pipapo.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/net/netfilter/nft_set_pipapo.c b/net/netfilter/nft_set_pipapo.c
> index df8de5090246..09c3eedc879b 100644
> --- a/net/netfilter/nft_set_pipapo.c
> +++ b/net/netfilter/nft_set_pipapo.c
> @@ -2077,6 +2077,8 @@ static void nft_pipapo_remove(const struct net *net, const struct nft_set *set,
>  		rules_fx = rules_f0;
>  
>  		nft_pipapo_for_each_field(f, i, m) {
> +			bool last = i == m->field_count - 1;
> +
>  			if (!pipapo_match_field(f, start, rules_fx,
>  						match_start, match_end))
>  				break;
> @@ -2089,16 +2091,22 @@ static void nft_pipapo_remove(const struct net *net, const struct nft_set *set,
>  
>  			match_start += NFT_PIPAPO_GROUPS_PADDED_SIZE(f);
>  			match_end += NFT_PIPAPO_GROUPS_PADDED_SIZE(f);
> -		}
>  
> -		if (i == m->field_count) {
> -			priv->dirty = true;
> -			pipapo_drop(m, rulemap);
> -			return;
> +			if (last) {
> +				const struct nft_pipapo_elem *this = f->mt[rulemap[i].to].e;
> +
> +				if (this == e) {

To avoid this very long line, we could probably assign 'this' on a
separate line, or even just:

				if (f->mt[rulemap[i].to].e == e) {

which I find equally readable.

Either way,

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>

-- 
Stefano