Re: [PATCH linux-next] ipv4: Fix NULL deference in fnhe_remove_oldest()

[Eric Dumazet]

On Thu, Sep 2, 2021 at 9:32 AM Tim Gardner <tim.gardner@xxxxxxxxxxxxx> wrote:
>
> Coverity complains that linux-next commit 67d6d681e15b5 ("ipv4: make
> exception cache less predictible") neglected to check for NULL before
> dereferencing 'oldest'. It appears to be possible to fall through the for
> loop without ever setting 'oldest'.

Coverity is wrong.

 fnhe_remove_oldest() is only called when there are at least 6 items
in the list.

There is no way oldest could be NULL, or that oldest_p could contain garbage.

>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
> Cc: David Ahern <dsahern@xxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Tim Gardner <tim.gardner@xxxxxxxxxxxxx>
> ---
>  net/ipv4/route.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index d6899ab5fb39..e85026591a09 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -603,9 +603,11 @@ static void fnhe_remove_oldest(struct fnhe_hash_bucket *hash)
>                         oldest_p = fnhe_p;
>                 }
>         }
> -       fnhe_flush_routes(oldest);
> -       *oldest_p = oldest->fnhe_next;
> -       kfree_rcu(oldest, rcu);
> +       if (oldest) {
> +               fnhe_flush_routes(oldest);
> +               *oldest_p = oldest->fnhe_next;
> +               kfree_rcu(oldest, rcu);
> +       }
>  }
>
>  static u32 fnhe_hashfun(__be32 daddr)
> --
> 2.33.0
>