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

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

[Tim Gardner]

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'.

Cc: Eric Dumazet <edumazet@google.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
---
 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

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@canonical.com> 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@google.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: netdev@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
> ---
>  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
>

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

[David Ahern]

On 9/2/21 9:38 AM, Eric Dumazet wrote:
> On Thu, Sep 2, 2021 at 9:32 AM Tim Gardner <tim.gardner@canonical.com> 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.
> 

+1