[net] packet: avoid out of bounds read in round robin fanout

From: Willem de Bruijn <willemb@google.com>

PACKET_FANOUT_LB computes f->rr_cur such that it is modulo
f->num_members. It returns the old value unconditionally, but
f->num_members may have changed since the last store. This can be
fixed with

  -        return cur
  +        return cur < num ? : 0;

When modifying the logic, simplify it further by replacing the loop
with an unconditional atomic increment.

Fixes: dc99f600698d ("packet: Add fanout support.")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 net/packet/af_packet.c | 19 ++-----------------
 1 file changed, 2 insertions(+), 17 deletions(-)

On Tue, Jun 16, 2015 at 5:07 PM, Willem de Bruijn <willemb@google.com> wrote:
> From: Willem de Bruijn <willemb@google.com>
>
> PACKET_FANOUT_LB computes f->rr_cur such that it is modulo
> f->num_members. It returns the old value unconditionally, but
> f->num_members may have changed since the last store. This can be
> fixed with
>
>   -        return cur
>   +        return cur < num ? : 0;

Well, that test is bad. Should be return cur < num ? cur : 0. But the
patch is more concise, anyway.

>
> When modifying the logic, simplify it further by replacing the loop
> with an unconditional atomic increment.
>
> Fixes: dc99f600698d ("packet: Add fanout support.")
> Suggested-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Willem de Bruijn <willemb@google.com>
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Hello.

On 6/17/2015 12:07 AM, Willem de Bruijn wrote:

> From: Willem de Bruijn <willemb@google.com>

> PACKET_FANOUT_LB computes f->rr_cur such that it is modulo
> f->num_members. It returns the old value unconditionally, but
> f->num_members may have changed since the last store. This can be
> fixed with

>    -        return cur
>    +        return cur < num ? : 0;

> When modifying the logic, simplify it further by replacing the loop
> with an unconditional atomic increment.

> Fixes: dc99f600698d ("packet: Add fanout support.")
> Suggested-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Willem de Bruijn <willemb@google.com>
> ---
>   net/packet/af_packet.c | 19 ++-----------------
>   1 file changed, 2 insertions(+), 17 deletions(-)

> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index b5989c6..efd35e8 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
[...]
> @@ -1293,13 +1283,8 @@ static unsigned int fanout_demux_lb(struct packet_fanout *f,
>   				    struct sk_buff *skb,
>   				    unsigned int num)
>   {
> -	int cur, old;
> -
> -	cur = atomic_read(&f->rr_cur);
> -	while ((old = atomic_cmpxchg(&f->rr_cur, cur,
> -				     fanout_rr_next(f, num))) != cur)
> -		cur = old;
> -	return cur;
> +	unsigned int val = atomic_inc_return(&f->rr_cur);

    Please insert an empty line after declaration, as it was before your patch.

> +	return val % num;
>   }
[...]

WBR, Sergei

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