[bpf-next,V12,2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx

Message ID 161098885996.108067.14467274374916086727.stgit@firesoul
State New
Headers show
Series
  • bpf: New approach for BPF MTU handling
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Commit Message

Jesper Dangaard Brouer Jan. 18, 2021, 4:54 p.m.
BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
by adjusting fib_params 'tot_len' with the packet length plus the expected
encap size. (Just like the bpf_check_mtu helper supports). He discovered
that for SKB ctx the param->tot_len was not used, instead skb->len was used
(via MTU check in is_skb_forwardable() that checks against netdev MTU).

Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.
zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU
check is done like XDP code-path, which checks against FIB-dst MTU.

V10:
- Use same method as XDP for 'tot_len' MTU check

Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
Reported-by: Carlo Carraro <colrack@gmail.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/filter.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

Comments

Daniel Borkmann Jan. 23, 2021, 1:47 a.m. | #1
On 1/18/21 5:54 PM, Jesper Dangaard Brouer wrote:
> BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use

> bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,

> by adjusting fib_params 'tot_len' with the packet length plus the expected

> encap size. (Just like the bpf_check_mtu helper supports). He discovered

> that for SKB ctx the param->tot_len was not used, instead skb->len was used

> (via MTU check in is_skb_forwardable() that checks against netdev MTU).

> 

> Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.

> zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU

> check is done like XDP code-path, which checks against FIB-dst MTU.

> 

> V10:

> - Use same method as XDP for 'tot_len' MTU check

> 

> Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")

> Reported-by: Carlo Carraro <colrack@gmail.com>

> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

> ---

>   net/core/filter.c |   13 ++++++++++---

>   1 file changed, 10 insertions(+), 3 deletions(-)

> 

> diff --git a/net/core/filter.c b/net/core/filter.c

> index 5beadd659091..d5e6f395cf64 100644

> --- a/net/core/filter.c

> +++ b/net/core/filter.c

> @@ -5569,6 +5569,7 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,

>   {

>   	struct net *net = dev_net(skb->dev);

>   	int rc = -EAFNOSUPPORT;

> +	bool check_mtu = false;

>   

>   	if (plen < sizeof(*params))

>   		return -EINVAL;

> @@ -5576,22 +5577,28 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,

>   	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))

>   		return -EINVAL;

>   

> +	if (params->tot_len)

> +		check_mtu = true;

> +

>   	switch (params->family) {

>   #if IS_ENABLED(CONFIG_INET)

>   	case AF_INET:

> -		rc = bpf_ipv4_fib_lookup(net, params, flags, false);

> +		rc = bpf_ipv4_fib_lookup(net, params, flags, check_mtu);

>   		break;

>   #endif

>   #if IS_ENABLED(CONFIG_IPV6)

>   	case AF_INET6:

> -		rc = bpf_ipv6_fib_lookup(net, params, flags, false);

> +		rc = bpf_ipv6_fib_lookup(net, params, flags, check_mtu);

>   		break;

>   #endif

>   	}

>   

> -	if (!rc) {

> +	if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {

>   		struct net_device *dev;

>   

> +		/* When tot_len isn't provided by user,

> +		 * check skb against net_device MTU

> +		 */

>   		dev = dev_get_by_index_rcu(net, params->ifindex);

>   		if (!is_skb_forwardable(dev, skb))

>   			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;


Btw, looking at some of the old feedback, looks like [0] got missed somehow. Would
be nice if we could simplify this rather ugly bit with refetching dev for tc.

   [0] https://lore.kernel.org/bpf/f959017b-5d3c-5cdb-a016-c467a3c9a2fc@iogearbox.net/
       https://lore.kernel.org/bpf/f8ff26f0-b1b6-6dd1-738d-4c592a8efdb0@gmail.com/
Jesper Dangaard Brouer Jan. 25, 2021, 3:58 p.m. | #2
On Sat, 23 Jan 2021 02:47:28 +0100
Daniel Borkmann <daniel@iogearbox.net> wrote:

> On 1/18/21 5:54 PM, Jesper Dangaard Brouer wrote:

> > BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use

> > bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,

> > by adjusting fib_params 'tot_len' with the packet length plus the expected

> > encap size. (Just like the bpf_check_mtu helper supports). He discovered

> > that for SKB ctx the param->tot_len was not used, instead skb->len was used

> > (via MTU check in is_skb_forwardable() that checks against netdev MTU).

> > 

> > Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.

> > zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU

> > check is done like XDP code-path, which checks against FIB-dst MTU.

> > 

> > V10:

> > - Use same method as XDP for 'tot_len' MTU check

> > 

> > Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")

> > Reported-by: Carlo Carraro <colrack@gmail.com>

> > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

> > ---

> >   net/core/filter.c |   13 ++++++++++---

> >   1 file changed, 10 insertions(+), 3 deletions(-)

> > 

> > diff --git a/net/core/filter.c b/net/core/filter.c

> > index 5beadd659091..d5e6f395cf64 100644

> > --- a/net/core/filter.c

> > +++ b/net/core/filter.c

> > @@ -5569,6 +5569,7 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,

> >   {

> >   	struct net *net = dev_net(skb->dev);

> >   	int rc = -EAFNOSUPPORT;

> > +	bool check_mtu = false;

> >   

> >   	if (plen < sizeof(*params))

> >   		return -EINVAL;

> > @@ -5576,22 +5577,28 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,

> >   	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))

> >   		return -EINVAL;

> >   

> > +	if (params->tot_len)

> > +		check_mtu = true;

> > +

> >   	switch (params->family) {

> >   #if IS_ENABLED(CONFIG_INET)

> >   	case AF_INET:

> > -		rc = bpf_ipv4_fib_lookup(net, params, flags, false);

> > +		rc = bpf_ipv4_fib_lookup(net, params, flags, check_mtu);

> >   		break;

> >   #endif

> >   #if IS_ENABLED(CONFIG_IPV6)

> >   	case AF_INET6:

> > -		rc = bpf_ipv6_fib_lookup(net, params, flags, false);

> > +		rc = bpf_ipv6_fib_lookup(net, params, flags, check_mtu);

> >   		break;

> >   #endif

> >   	}

> >   

> > -	if (!rc) {

> > +	if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {

> >   		struct net_device *dev;

> >   

> > +		/* When tot_len isn't provided by user,

> > +		 * check skb against net_device MTU

> > +		 */

> >   		dev = dev_get_by_index_rcu(net, params->ifindex);

> >   		if (!is_skb_forwardable(dev, skb))

> >   			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;  

> 

> Btw, looking at some of the old feedback, looks like [0] got missed somehow. Would

> be nice if we could simplify this rather ugly bit with refetching dev for tc.

> 

>    [0] https://lore.kernel.org/bpf/f959017b-5d3c-5cdb-a016-c467a3c9a2fc@iogearbox.net/

>        https://lore.kernel.org/bpf/f8ff26f0-b1b6-6dd1-738d-4c592a8efdb0@gmail.com/


I have tried to incorporate the ideas from [0].  If you notice this
code path is only called in-case params->tot_len is zero (!check_mtu).
Then if params->tot_len does contain something, then the check_mtu code
path of bpf_ipv4_fib_lookup() and bpf_ipv6_fib_lookup().

I agree, that it is ugly refetching dev.  I'll look at moving the
lookup dev_get_by_index_rcu() call one level-up, as both
bpf_ipv4_fib_lookup() and bpf_ipv6_fib_lookup() does this call. (I
cannot move the check into the functions due skb is not avail in XDP
case)

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

Patch

diff --git a/net/core/filter.c b/net/core/filter.c
index 5beadd659091..d5e6f395cf64 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5569,6 +5569,7 @@  BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 {
 	struct net *net = dev_net(skb->dev);
 	int rc = -EAFNOSUPPORT;
+	bool check_mtu = false;
 
 	if (plen < sizeof(*params))
 		return -EINVAL;
@@ -5576,22 +5577,28 @@  BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
 		return -EINVAL;
 
+	if (params->tot_len)
+		check_mtu = true;
+
 	switch (params->family) {
 #if IS_ENABLED(CONFIG_INET)
 	case AF_INET:
-		rc = bpf_ipv4_fib_lookup(net, params, flags, false);
+		rc = bpf_ipv4_fib_lookup(net, params, flags, check_mtu);
 		break;
 #endif
 #if IS_ENABLED(CONFIG_IPV6)
 	case AF_INET6:
-		rc = bpf_ipv6_fib_lookup(net, params, flags, false);
+		rc = bpf_ipv6_fib_lookup(net, params, flags, check_mtu);
 		break;
 #endif
 	}
 
-	if (!rc) {
+	if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {
 		struct net_device *dev;
 
+		/* When tot_len isn't provided by user,
+		 * check skb against net_device MTU
+		 */
 		dev = dev_get_by_index_rcu(net, params->ifindex);
 		if (!is_skb_forwardable(dev, skb))
 			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;