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

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

[Willem de Bruijn]

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
@@ -1272,16 +1272,6 @@ static void packet_sock_destruct(struct sock *sk)
 	sk_refcnt_debug_dec(sk);
 }
 
-static int fanout_rr_next(struct packet_fanout *f, unsigned int num)
-{
-	int x = atomic_read(&f->rr_cur) + 1;
-
-	if (x >= num)
-		x = 0;
-
-	return x;
-}
-
 static unsigned int fanout_demux_hash(struct packet_fanout *f,
 				      struct sk_buff *skb,
 				      unsigned int num)
@@ -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);
+	return val % num;
 }
 
 static unsigned int fanout_demux_cpu(struct packet_fanout *f,
-- 
2.2.0.rc0.207.ga3a616c

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

[Willem de Bruijn]

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>

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

[Sergei Shtylyov]

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

[PATCH net v2] packet: avoid out of bounds read in round robin fanout

[Willem de Bruijn]

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. Ensure
that the return value is always < num.

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 | 18 ++----------------
 1 file changed, 2 insertions(+), 16 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index b5989c6..104f902 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1272,16 +1272,6 @@ static void packet_sock_destruct(struct sock *sk)
 	sk_refcnt_debug_dec(sk);
 }
 
-static int fanout_rr_next(struct packet_fanout *f, unsigned int num)
-{
-	int x = atomic_read(&f->rr_cur) + 1;
-
-	if (x >= num)
-		x = 0;
-
-	return x;
-}
-
 static unsigned int fanout_demux_hash(struct packet_fanout *f,
 				      struct sk_buff *skb,
 				      unsigned int num)
@@ -1293,13 +1283,9 @@ static unsigned int fanout_demux_lb(struct packet_fanout *f,
 				    struct sk_buff *skb,
 				    unsigned int num)
 {
-	int cur, old;
+	unsigned int val = atomic_inc_return(&f->rr_cur);
 
-	cur = atomic_read(&f->rr_cur);
-	while ((old = atomic_cmpxchg(&f->rr_cur, cur,
-				     fanout_rr_next(f, num))) != cur)
-		cur = old;
-	return cur;
+	return val % num;
 }
 
 static unsigned int fanout_demux_cpu(struct packet_fanout *f,
-- 
2.4.3.573.g4eafbef

Re: [PATCH net v2] packet: avoid out of bounds read in round robin fanout

[Eric Dumazet]

On Wed, 2015-06-17 at 15:59 -0400, 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. Ensure
> that the return value is always < num.
> 
> 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>
> ---

Acked-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH net v2] packet: avoid out of bounds read in round robin fanout

[David Miller]

From: Willem de Bruijn <willemb@google.com>
Date: Wed, 17 Jun 2015 15:59:34 -0400

> 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. Ensure
> that the return value is always < num.
> 
> 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>

Applied and queued up for -stable, thanks.