Before commit 5ee759cda51b ("l2tp: use standard API for warning log
messages"), it was possible for userspace applications to use their own
control protocols on the backing sockets of an L2TP kernel device, and as
long as a packet didn't look like a proper L2TP data packet, it would be
passed up to userspace just fine.
After the mentioned change, this approach would lead to significant log
spam, as the previously hidden warnings are now shown by default. Not
even setting the T flag on the custom control packets is sufficient to
surpress these warnings, as packet length and L2TP version are checked
before the T flag.
Reduce all warnings debug level when packets are passed to userspace.
Fixes: 5ee759cda51b ("l2tp: use standard API for warning log messages")
Signed-off-by: Matthias Schiffer <redacted>
---
I'm unsure what to do about the pr_warn_ratelimited() in
l2tp_recv_common(). It feels wrong to me that an incoming network packet
can trigger a kernel message above debug level at all, so maybe they
should be downgraded as well? I believe the only reason these were ever
warnings is that they were not shown by default.
net/l2tp/l2tp_core.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
@@ -809,8 +809,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Short packet? */if(!pskb_may_pull(skb,L2TP_HDR_SIZE_MAX)){-pr_warn_ratelimited("%s: recv short packet (len=%d)\n",-tunnel->name,skb->len);+pr_debug_ratelimited("%s: recv short packet (len=%d)\n",+tunnel->name,skb->len);gotoerror;}
@@ -824,8 +824,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Check protocol version */version=hdrflags&L2TP_HDR_VER_MASK;if(version!=tunnel->version){-pr_warn_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",-tunnel->name,version,tunnel->version);+pr_debug_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",+tunnel->name,version,tunnel->version);gotoerror;}
@@ -863,8 +863,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)l2tp_session_dec_refcount(session);/* Not found? Pass to userspace to deal with */-pr_warn_ratelimited("%s: no session found (%u/%u). Passing up.\n",-tunnel->name,tunnel_id,session_id);+pr_debug_ratelimited("%s: no session found (%u/%u). Passing up.\n",+tunnel->name,tunnel_id,session_id);gotoerror;}
From: Tom Parkin <hidden> Date: 2021-02-19 20:19:35
Hi Matthias,
Thanks for your patch!
On Fri, Feb 19, 2021 at 20:06:15 +0100, Matthias Schiffer wrote:
Before commit 5ee759cda51b ("l2tp: use standard API for warning log
messages"), it was possible for userspace applications to use their own
control protocols on the backing sockets of an L2TP kernel device, and as
long as a packet didn't look like a proper L2TP data packet, it would be
passed up to userspace just fine.
Hum. I appreciate we're now logging where we previously were not, but
I would say these warning messages are valid.
It's still perfectly possible to use the L2TP socket for the L2TP control
protocol: packets per the RFCs won't trigger these warnings to the
best of my knowledge, although I'm happy to be proven wrong!
I wonder whether your application is sending non-L2TP packets over the
L2TP socket? Could you describe the usecase?
After the mentioned change, this approach would lead to significant log
spam, as the previously hidden warnings are now shown by default. Not
even setting the T flag on the custom control packets is sufficient to
surpress these warnings, as packet length and L2TP version are checked
before the T flag.
Possibly we could sidestep some of these warnings by moving the T flag
check further up in the function.
The code would need to pull the first byte of the header, check the type
bit, and skip further processing if the bit was set. Otherwise go on to
pull the rest of the header.
I think I'd prefer this approach assuming the warnings are not
triggered by valid L2TP messages.
quoted hunk
Reduce all warnings debug level when packets are passed to userspace.
Fixes: 5ee759cda51b ("l2tp: use standard API for warning log messages")
Signed-off-by: Matthias Schiffer <redacted>
---
I'm unsure what to do about the pr_warn_ratelimited() in
l2tp_recv_common(). It feels wrong to me that an incoming network packet
can trigger a kernel message above debug level at all, so maybe they
should be downgraded as well? I believe the only reason these were ever
warnings is that they were not shown by default.
net/l2tp/l2tp_core.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
@@ -809,8 +809,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Short packet? */if(!pskb_may_pull(skb,L2TP_HDR_SIZE_MAX)){-pr_warn_ratelimited("%s: recv short packet (len=%d)\n",-tunnel->name,skb->len);+pr_debug_ratelimited("%s: recv short packet (len=%d)\n",+tunnel->name,skb->len);gotoerror;}
@@ -824,8 +824,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Check protocol version */version=hdrflags&L2TP_HDR_VER_MASK;if(version!=tunnel->version){-pr_warn_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",-tunnel->name,version,tunnel->version);+pr_debug_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",+tunnel->name,version,tunnel->version);gotoerror;}
@@ -863,8 +863,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)l2tp_session_dec_refcount(session);/* Not found? Pass to userspace to deal with */-pr_warn_ratelimited("%s: no session found (%u/%u). Passing up.\n",-tunnel->name,tunnel_id,session_id);+pr_debug_ratelimited("%s: no session found (%u/%u). Passing up.\n",+tunnel->name,tunnel_id,session_id);gotoerror;}
Hi Matthias,
Thanks for your patch!
On Fri, Feb 19, 2021 at 20:06:15 +0100, Matthias Schiffer wrote:
quoted
Before commit 5ee759cda51b ("l2tp: use standard API for warning log
messages"), it was possible for userspace applications to use their own
control protocols on the backing sockets of an L2TP kernel device, and as
long as a packet didn't look like a proper L2TP data packet, it would be
passed up to userspace just fine.
Hum. I appreciate we're now logging where we previously were not, but
I would say these warning messages are valid.
It's still perfectly possible to use the L2TP socket for the L2TP control
protocol: packets per the RFCs won't trigger these warnings to the
best of my knowledge, although I'm happy to be proven wrong!
I wonder whether your application is sending non-L2TP packets over the
L2TP socket? Could you describe the usecase?
I'm the developer of the UDP-based VPN/tunnel application fastd [1]. This
is currently a pure userspace implementation, supporting both encrypted and
unencrypted tunnels, with a protocol that doesn't look anything like L2TP.
To improve performance of unencrypted tunnels, I'm looking into using L2TP
to offload the data plane to the kernel. Whether to make use of this would
be negotiated in a session handshake (that is also used for key exchange in
encrypted mode).
As the (IPv4) internet is stupid and everything is NATted, and I even want
to support broken NAT routers that somehow break UDP hole punching, I use
only a single socket for both control (handshake) and data packets.
quoted
After the mentioned change, this approach would lead to significant log
spam, as the previously hidden warnings are now shown by default. Not
even setting the T flag on the custom control packets is sufficient to
surpress these warnings, as packet length and L2TP version are checked
before the T flag.
Possibly we could sidestep some of these warnings by moving the T flag
check further up in the function.
The code would need to pull the first byte of the header, check the type
bit, and skip further processing if the bit was set. Otherwise go on to
pull the rest of the header.
I think I'd prefer this approach assuming the warnings are not
triggered by valid L2TP messages.
This will not be sufficient for my usecase: To stay compatible with older
versions of fastd, I can't set the T flag in the first packet of the
handshake, as it won't be known whether the peer has a new enough fastd
version to understand packets that have this bit set. Luckily, the second
handshake byte is always 0 in fastd's protocol, so these packets fail the
tunnel version check and are passed to userspace regardless.
I'm aware that this usecase is far outside of the original intentions of
the code and can only be described as a hack, but I still consider this a
regression in the kernel, as it was working fine in the past, without
visible warnings.
[1] https://github.com/NeoRaider/fastd/
quoted
Reduce all warnings debug level when packets are passed to userspace.
Fixes: 5ee759cda51b ("l2tp: use standard API for warning log messages")
Signed-off-by: Matthias Schiffer <redacted>
quoted
---
I'm unsure what to do about the pr_warn_ratelimited() in
l2tp_recv_common(). It feels wrong to me that an incoming network packet
can trigger a kernel message above debug level at all, so maybe they
should be downgraded as well? I believe the only reason these were ever
warnings is that they were not shown by default.
net/l2tp/l2tp_core.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
@@ -809,8 +809,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Short packet? */if(!pskb_may_pull(skb,L2TP_HDR_SIZE_MAX)){-pr_warn_ratelimited("%s: recv short packet (len=%d)\n",-tunnel->name,skb->len);+pr_debug_ratelimited("%s: recv short packet (len=%d)\n",+tunnel->name,skb->len);gotoerror;}
@@ -824,8 +824,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Check protocol version */version=hdrflags&L2TP_HDR_VER_MASK;if(version!=tunnel->version){-pr_warn_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",-tunnel->name,version,tunnel->version);+pr_debug_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",+tunnel->name,version,tunnel->version);gotoerror;}
@@ -863,8 +863,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)l2tp_session_dec_refcount(session);/* Not found? Pass to userspace to deal with */-pr_warn_ratelimited("%s: no session found (%u/%u). Passing up.\n",-tunnel->name,tunnel_id,session_id);+pr_debug_ratelimited("%s: no session found (%u/%u). Passing up.\n",+tunnel->name,tunnel_id,session_id);gotoerror;}
From: Tom Parkin <hidden> Date: 2021-02-22 11:50:21
On Sat, Feb 20, 2021 at 10:56:33 +0100, Matthias Schiffer wrote:
On 2/19/21 9:12 PM, Tom Parkin wrote:
quoted
On Fri, Feb 19, 2021 at 20:06:15 +0100, Matthias Schiffer wrote:
quoted
Before commit 5ee759cda51b ("l2tp: use standard API for warning log
messages"), it was possible for userspace applications to use their own
control protocols on the backing sockets of an L2TP kernel device, and as
long as a packet didn't look like a proper L2TP data packet, it would be
passed up to userspace just fine.
Hum. I appreciate we're now logging where we previously were not, but
I would say these warning messages are valid.
It's still perfectly possible to use the L2TP socket for the L2TP control
protocol: packets per the RFCs won't trigger these warnings to the
best of my knowledge, although I'm happy to be proven wrong!
I wonder whether your application is sending non-L2TP packets over the
L2TP socket? Could you describe the usecase?
I'm the developer of the UDP-based VPN/tunnel application fastd [1]. This is
currently a pure userspace implementation, supporting both encrypted and
unencrypted tunnels, with a protocol that doesn't look anything like L2TP.
To improve performance of unencrypted tunnels, I'm looking into using L2TP
to offload the data plane to the kernel. Whether to make use of this would
be negotiated in a session handshake (that is also used for key exchange in
encrypted mode).
As the (IPv4) internet is stupid and everything is NATted, and I even want
to support broken NAT routers that somehow break UDP hole punching, I use
only a single socket for both control (handshake) and data packets.
Thanks for describing the usecase a bit more. It looks an interesting
project.
To be honest I'm not sure the L2TP subsystem is a good match for
fastd's datapath offload though.
This is for the following reasons:
Firstly, the warnings referenced in your patch are early in the L2TP recv
path, called shortly after our hook into the UDP recv code.
So at this point, I don't believe the L2TP subsystem is really buying you
anything over a plain UPD recv.
Now, I'm perhaps reading too much into what you've said, but I imagine
that fastd using the L2TP tunnel context is just a first step. I
assume the end goal for datapath offload would be to use the L2TP
pseudowire code in order to have session data appear from e.g. a
virtual Ethernet interface. That way you get to avoid copying data
to userspace, and then stuffing it back into the kernel via. tun/tap.
And that makes sense to me as a goal!
However, if that is indeed the goal, I really can't see it working
without fastd's protocol being modified to look like L2TP. (I also,
btw, can't see it working without some kind of kernel-space L2TP
subsystem support for fastd's various encryption protocols, but that's
another matter).
If you take a look at the session recv datapath from l2tp_recv_common
onwards you'll see that there is a lot of code you have to avoid
confusing in l2tp_core.c alone, even before you get into any
pseudowire-specifics. I can't see that being possible with fastd's
current data packet format.
In summary, I think at this point in the L2TP recv path a normal UDP
socket would serve you just as well, and I can't see the L2TP subsystem
being useful as a data offload mechanism for fastd in the future
without effectively changing fastd's packet format to look like L2TP.
Secondly, given the above (and apologies if I've missed the mark); I
really wouldn't want to encourage you to use the L2TP subsystem for
future fastd improvements.
From fastd's perspective it is IMO a bad idea, since it would be easy
for a future (perfectly valid) change in the L2TP code to accidentally
break fastd. And next time it might not be some easily-tweaked thing
like logging levels, but rather e.g. a security fix or bug fix which
cannot be undone.
From the L2TP subsystem's perspective it is a bad idea, since by
encouraging fastd to use the L2TP code, we end up hampering future L2TP
development in order to support a project which doesn't actually use
the L2TP protocol at all.
In the hope of being more constructive -- have you considered whether
tc and/or ebpf could be used for fastd? As more generic mechanisms I
think you might get on better with these than trying to twist the L2TP
code's arm :-)
quoted
quoted
After the mentioned change, this approach would lead to significant log
spam, as the previously hidden warnings are now shown by default. Not
even setting the T flag on the custom control packets is sufficient to
surpress these warnings, as packet length and L2TP version are checked
before the T flag.
Possibly we could sidestep some of these warnings by moving the T flag
check further up in the function.
The code would need to pull the first byte of the header, check the type
bit, and skip further processing if the bit was set. Otherwise go on to
pull the rest of the header.
I think I'd prefer this approach assuming the warnings are not
triggered by valid L2TP messages.
This will not be sufficient for my usecase: To stay compatible with older
versions of fastd, I can't set the T flag in the first packet of the
handshake, as it won't be known whether the peer has a new enough fastd
version to understand packets that have this bit set. Luckily, the second
handshake byte is always 0 in fastd's protocol, so these packets fail the
tunnel version check and are passed to userspace regardless.
I'm aware that this usecase is far outside of the original intentions of the
code and can only be described as a hack, but I still consider this a
regression in the kernel, as it was working fine in the past, without
visible warnings.
I'm sorry, but for the reasons stated above I disagree about it being
a regression.
Reduce all warnings debug level when packets are passed to userspace.
Fixes: 5ee759cda51b ("l2tp: use standard API for warning log messages")
Signed-off-by: Matthias Schiffer <redacted>
quoted
quoted
---
I'm unsure what to do about the pr_warn_ratelimited() in
l2tp_recv_common(). It feels wrong to me that an incoming network packet
can trigger a kernel message above debug level at all, so maybe they
should be downgraded as well? I believe the only reason these were ever
warnings is that they were not shown by default.
net/l2tp/l2tp_core.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
@@ -809,8 +809,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Short packet? */if(!pskb_may_pull(skb,L2TP_HDR_SIZE_MAX)){-pr_warn_ratelimited("%s: recv short packet (len=%d)\n",-tunnel->name,skb->len);+pr_debug_ratelimited("%s: recv short packet (len=%d)\n",+tunnel->name,skb->len);gotoerror;}
@@ -824,8 +824,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Check protocol version */version=hdrflags&L2TP_HDR_VER_MASK;if(version!=tunnel->version){-pr_warn_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",-tunnel->name,version,tunnel->version);+pr_debug_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",+tunnel->name,version,tunnel->version);gotoerror;}
@@ -863,8 +863,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)l2tp_session_dec_refcount(session);/* Not found? Pass to userspace to deal with */-pr_warn_ratelimited("%s: no session found (%u/%u). Passing up.\n",-tunnel->name,tunnel_id,session_id);+pr_debug_ratelimited("%s: no session found (%u/%u). Passing up.\n",+tunnel->name,tunnel_id,session_id);gotoerror;}
On Sat, Feb 20, 2021 at 10:56:33 +0100, Matthias Schiffer wrote:
quoted
On 2/19/21 9:12 PM, Tom Parkin wrote:
quoted
On Fri, Feb 19, 2021 at 20:06:15 +0100, Matthias Schiffer wrote:
quoted
Before commit 5ee759cda51b ("l2tp: use standard API for warning log
messages"), it was possible for userspace applications to use their own
control protocols on the backing sockets of an L2TP kernel device, and as
long as a packet didn't look like a proper L2TP data packet, it would be
passed up to userspace just fine.
Hum. I appreciate we're now logging where we previously were not, but
I would say these warning messages are valid.
It's still perfectly possible to use the L2TP socket for the L2TP control
protocol: packets per the RFCs won't trigger these warnings to the
best of my knowledge, although I'm happy to be proven wrong!
I wonder whether your application is sending non-L2TP packets over the
L2TP socket? Could you describe the usecase?
I'm the developer of the UDP-based VPN/tunnel application fastd [1]. This is
currently a pure userspace implementation, supporting both encrypted and
unencrypted tunnels, with a protocol that doesn't look anything like L2TP.
To improve performance of unencrypted tunnels, I'm looking into using L2TP
to offload the data plane to the kernel. Whether to make use of this would
be negotiated in a session handshake (that is also used for key exchange in
encrypted mode).
As the (IPv4) internet is stupid and everything is NATted, and I even want
to support broken NAT routers that somehow break UDP hole punching, I use
only a single socket for both control (handshake) and data packets.
Thanks for describing the usecase a bit more. It looks an interesting
project.
To be honest I'm not sure the L2TP subsystem is a good match for
fastd's datapath offload though.
This is for the following reasons:
Firstly, the warnings referenced in your patch are early in the L2TP recv
path, called shortly after our hook into the UDP recv code.
So at this point, I don't believe the L2TP subsystem is really buying you
anything over a plain UPD recv.
Now, I'm perhaps reading too much into what you've said, but I imagine
that fastd using the L2TP tunnel context is just a first step. I
assume the end goal for datapath offload would be to use the L2TP
pseudowire code in order to have session data appear from e.g. a
virtual Ethernet interface. That way you get to avoid copying data
to userspace, and then stuffing it back into the kernel via. tun/tap.
And that makes sense to me as a goal!
That is indeed what I want to achieve.
However, if that is indeed the goal, I really can't see it working
without fastd's protocol being modified to look like L2TP. (I also,
btw, can't see it working without some kind of kernel-space L2TP
subsystem support for fastd's various encryption protocols, but that's
another matter).
Only unencrypted connections would be offloaded.
fastd's data protocol will be modified to use L2TP Ethernet pseudowire as
specified by the RFC (I actually finished the userspace implementation of
this yesterday, in branch method-l2tp for now). Two peers can negotiate the
protocol to use (called "method" in fastd terms) in the session handshake.
A session would only be offloaded to kernel-space L2TP when both sides
agree that they indeed want to use the L2TP method; otherwise fastd will
continue to use TUN/TAP.
The only part of the fastd protocol that I can't modify arbitrarily is the
first packet of the handshake, as it must be compatible with older versions
of fastd. There may be a point when I can set the T flag in handshakes
unconditionally, but that would be 3~5 years in the future.
If you take a look at the session recv datapath from l2tp_recv_common
onwards you'll see that there is a lot of code you have to avoid
confusing in l2tp_core.c alone, even before you get into any
pseudowire-specifics. I can't see that being possible with fastd's
current data packet format >
In summary, I think at this point in the L2TP recv path a normal UDP
socket would serve you just as well, and I can't see the L2TP subsystem
being useful as a data offload mechanism for fastd in the future
without effectively changing fastd's packet format to look like L2TP.
Secondly, given the above (and apologies if I've missed the mark); I
really wouldn't want to encourage you to use the L2TP subsystem for
future fastd improvements.
From fastd's perspective it is IMO a bad idea, since it would be easy
for a future (perfectly valid) change in the L2TP code to accidentally
break fastd. And next time it might not be some easily-tweaked thing
like logging levels, but rather e.g. a security fix or bug fix which
cannot be undone.
From the L2TP subsystem's perspective it is a bad idea, since by
encouraging fastd to use the L2TP code, we end up hampering future L2TP
development in order to support a project which doesn't actually use
the L2TP protocol at all.
As explained above, this only concerns fastd's handshake format. As long as
no new L2TP version accepts 0 as its "Version" field and such packets
continue to passed to userspace even without the T flag, fastd would be fine.
In the hope of being more constructive -- have you considered whether
tc and/or ebpf could be used for fastd? As more generic mechanisms I
think you might get on better with these than trying to twist the L2TP
code's arm :-)
(e)BPF might actually be an option. I will look into this.
quoted
quoted
quoted
After the mentioned change, this approach would lead to significant log
spam, as the previously hidden warnings are now shown by default. Not
even setting the T flag on the custom control packets is sufficient to
surpress these warnings, as packet length and L2TP version are checked
before the T flag.
Possibly we could sidestep some of these warnings by moving the T flag
check further up in the function.
The code would need to pull the first byte of the header, check the type
bit, and skip further processing if the bit was set. Otherwise go on to
pull the rest of the header.
I think I'd prefer this approach assuming the warnings are not
triggered by valid L2TP messages. >>
This will not be sufficient for my usecase: To stay compatible with older
versions of fastd, I can't set the T flag in the first packet of the
handshake, as it won't be known whether the peer has a new enough fastd
version to understand packets that have this bit set. Luckily, the second
handshake byte is always 0 in fastd's protocol, so these packets fail the
tunnel version check and are passed to userspace regardless.
I'm aware that this usecase is far outside of the original intentions of the
code and can only be described as a hack, but I still consider this a
regression in the kernel, as it was working fine in the past, without
visible warnings.
I'm sorry, but for the reasons stated above I disagree about it being
a regression.
Hmm, is it common for protocol implementations in the kernel to warn about
invalid packets they receive? While L2TP uses connected sockets and thus
usually no unrelated packets end up in the socket, a simple UDP port scan
originating from the configured remote address/port will trigger the "short
packet" warning now (nmap uses a zero-length payload for UDP scans by
default). Log spam caused by a malicous party might also be a concern.
Reduce all warnings debug level when packets are passed to userspace.
Fixes: 5ee759cda51b ("l2tp: use standard API for warning log messages")
Signed-off-by: Matthias Schiffer <redacted>
quoted
quoted
---
I'm unsure what to do about the pr_warn_ratelimited() in
l2tp_recv_common(). It feels wrong to me that an incoming network packet
can trigger a kernel message above debug level at all, so maybe they
should be downgraded as well? I believe the only reason these were ever
warnings is that they were not shown by default.
net/l2tp/l2tp_core.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
@@ -809,8 +809,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Short packet? */if(!pskb_may_pull(skb,L2TP_HDR_SIZE_MAX)){-pr_warn_ratelimited("%s: recv short packet (len=%d)\n",-tunnel->name,skb->len);+pr_debug_ratelimited("%s: recv short packet (len=%d)\n",+tunnel->name,skb->len);gotoerror;}
@@ -824,8 +824,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Check protocol version */version=hdrflags&L2TP_HDR_VER_MASK;if(version!=tunnel->version){-pr_warn_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",-tunnel->name,version,tunnel->version);+pr_debug_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",+tunnel->name,version,tunnel->version);gotoerror;}
@@ -863,8 +863,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)l2tp_session_dec_refcount(session);/* Not found? Pass to userspace to deal with */-pr_warn_ratelimited("%s: no session found (%u/%u). Passing up.\n",-tunnel->name,tunnel_id,session_id);+pr_debug_ratelimited("%s: no session found (%u/%u). Passing up.\n",+tunnel->name,tunnel_id,session_id);gotoerror;}
From: Jakub Kicinski <kuba@kernel.org> Date: 2021-02-22 22:32:36
On Mon, 22 Feb 2021 17:40:16 +0100 Matthias Schiffer wrote:
quoted
quoted
This will not be sufficient for my usecase: To stay compatible with older
versions of fastd, I can't set the T flag in the first packet of the
handshake, as it won't be known whether the peer has a new enough fastd
version to understand packets that have this bit set. Luckily, the second
handshake byte is always 0 in fastd's protocol, so these packets fail the
tunnel version check and are passed to userspace regardless.
I'm aware that this usecase is far outside of the original intentions of the
code and can only be described as a hack, but I still consider this a
regression in the kernel, as it was working fine in the past, without
visible warnings.
I'm sorry, but for the reasons stated above I disagree about it being
a regression.
Hmm, is it common for protocol implementations in the kernel to warn about
invalid packets they receive? While L2TP uses connected sockets and thus
usually no unrelated packets end up in the socket, a simple UDP port scan
originating from the configured remote address/port will trigger the "short
packet" warning now (nmap uses a zero-length payload for UDP scans by
default). Log spam caused by a malicous party might also be a concern.
Indeed, seems like appropriate counters would be a good fit here?
The prints are both potentially problematic for security and lossy.
From: Tom Parkin <hidden> Date: 2021-02-23 09:49:57
On Mon, Feb 22, 2021 at 14:31:38 -0800, Jakub Kicinski wrote:
On Mon, 22 Feb 2021 17:40:16 +0100 Matthias Schiffer wrote:
quoted
quoted
quoted
This will not be sufficient for my usecase: To stay compatible with older
versions of fastd, I can't set the T flag in the first packet of the
handshake, as it won't be known whether the peer has a new enough fastd
version to understand packets that have this bit set. Luckily, the second
handshake byte is always 0 in fastd's protocol, so these packets fail the
tunnel version check and are passed to userspace regardless.
I'm aware that this usecase is far outside of the original intentions of the
code and can only be described as a hack, but I still consider this a
regression in the kernel, as it was working fine in the past, without
visible warnings.
I'm sorry, but for the reasons stated above I disagree about it being
a regression.
Hmm, is it common for protocol implementations in the kernel to warn about
invalid packets they receive? While L2TP uses connected sockets and thus
usually no unrelated packets end up in the socket, a simple UDP port scan
originating from the configured remote address/port will trigger the "short
packet" warning now (nmap uses a zero-length payload for UDP scans by
default). Log spam caused by a malicous party might also be a concern.
Indeed, seems like appropriate counters would be a good fit here?
The prints are both potentially problematic for security and lossy.
On Mon, Feb 22, 2021 at 14:31:38 -0800, Jakub Kicinski wrote:
quoted
On Mon, 22 Feb 2021 17:40:16 +0100 Matthias Schiffer wrote:
quoted
quoted
quoted
This will not be sufficient for my usecase: To stay compatible with older
versions of fastd, I can't set the T flag in the first packet of the
handshake, as it won't be known whether the peer has a new enough fastd
version to understand packets that have this bit set. Luckily, the second
handshake byte is always 0 in fastd's protocol, so these packets fail the
tunnel version check and are passed to userspace regardless.
I'm aware that this usecase is far outside of the original intentions of the
code and can only be described as a hack, but I still consider this a
regression in the kernel, as it was working fine in the past, without
visible warnings.
I'm sorry, but for the reasons stated above I disagree about it being
a regression.
Hmm, is it common for protocol implementations in the kernel to warn about
invalid packets they receive? While L2TP uses connected sockets and thus
usually no unrelated packets end up in the socket, a simple UDP port scan
originating from the configured remote address/port will trigger the "short
packet" warning now (nmap uses a zero-length payload for UDP scans by
default). Log spam caused by a malicous party might also be a concern.
Indeed, seems like appropriate counters would be a good fit here?
The prints are both potentially problematic for security and lossy.
From: Tom Parkin <hidden> Date: 2021-02-23 09:47:30
On Mon, Feb 22, 2021 at 17:40:16 +0100, Matthias Schiffer wrote:
On 2/22/21 12:49 PM, Tom Parkin wrote:
quoted
On Sat, Feb 20, 2021 at 10:56:33 +0100, Matthias Schiffer wrote:
quoted
On 2/19/21 9:12 PM, Tom Parkin wrote:
quoted
On Fri, Feb 19, 2021 at 20:06:15 +0100, Matthias Schiffer wrote:
quoted
Before commit 5ee759cda51b ("l2tp: use standard API for warning log
messages"), it was possible for userspace applications to use their own
control protocols on the backing sockets of an L2TP kernel device, and as
long as a packet didn't look like a proper L2TP data packet, it would be
passed up to userspace just fine.
Hum. I appreciate we're now logging where we previously were not, but
I would say these warning messages are valid.
It's still perfectly possible to use the L2TP socket for the L2TP control
protocol: packets per the RFCs won't trigger these warnings to the
best of my knowledge, although I'm happy to be proven wrong!
I wonder whether your application is sending non-L2TP packets over the
L2TP socket? Could you describe the usecase?
I'm the developer of the UDP-based VPN/tunnel application fastd [1]. This is
currently a pure userspace implementation, supporting both encrypted and
unencrypted tunnels, with a protocol that doesn't look anything like L2TP.
To improve performance of unencrypted tunnels, I'm looking into using L2TP
to offload the data plane to the kernel. Whether to make use of this would
be negotiated in a session handshake (that is also used for key exchange in
encrypted mode).
As the (IPv4) internet is stupid and everything is NATted, and I even want
to support broken NAT routers that somehow break UDP hole punching, I use
only a single socket for both control (handshake) and data packets.
Thanks for describing the usecase a bit more. It looks an interesting
project.
To be honest I'm not sure the L2TP subsystem is a good match for
fastd's datapath offload though.
This is for the following reasons:
Firstly, the warnings referenced in your patch are early in the L2TP recv
path, called shortly after our hook into the UDP recv code.
So at this point, I don't believe the L2TP subsystem is really buying you
anything over a plain UPD recv.
Now, I'm perhaps reading too much into what you've said, but I imagine
that fastd using the L2TP tunnel context is just a first step. I
assume the end goal for datapath offload would be to use the L2TP
pseudowire code in order to have session data appear from e.g. a
virtual Ethernet interface. That way you get to avoid copying data
to userspace, and then stuffing it back into the kernel via. tun/tap.
And that makes sense to me as a goal!
That is indeed what I want to achieve.
quoted
However, if that is indeed the goal, I really can't see it working
without fastd's protocol being modified to look like L2TP. (I also,
btw, can't see it working without some kind of kernel-space L2TP
subsystem support for fastd's various encryption protocols, but that's
another matter).
Only unencrypted connections would be offloaded.
fastd's data protocol will be modified to use L2TP Ethernet pseudowire as
specified by the RFC (I actually finished the userspace implementation of
this yesterday, in branch method-l2tp for now). Two peers can negotiate the
protocol to use (called "method" in fastd terms) in the session handshake. A
session would only be offloaded to kernel-space L2TP when both sides agree
that they indeed want to use the L2TP method; otherwise fastd will continue
to use TUN/TAP.
The only part of the fastd protocol that I can't modify arbitrarily is the
first packet of the handshake, as it must be compatible with older versions
of fastd. There may be a point when I can set the T flag in handshakes
unconditionally, but that would be 3~5 years in the future.
I see. So the session would end up using L2TP headers, which seems
like it should be fine.
quoted
If you take a look at the session recv datapath from l2tp_recv_common
onwards you'll see that there is a lot of code you have to avoid
confusing in l2tp_core.c alone, even before you get into any
pseudowire-specifics. I can't see that being possible with fastd's
current data packet format >
In summary, I think at this point in the L2TP recv path a normal UDP
socket would serve you just as well, and I can't see the L2TP subsystem
being useful as a data offload mechanism for fastd in the future
without effectively changing fastd's packet format to look like L2TP.
Secondly, given the above (and apologies if I've missed the mark); I
really wouldn't want to encourage you to use the L2TP subsystem for
future fastd improvements.
From fastd's perspective it is IMO a bad idea, since it would be easy
for a future (perfectly valid) change in the L2TP code to accidentally
break fastd. And next time it might not be some easily-tweaked thing
like logging levels, but rather e.g. a security fix or bug fix which
cannot be undone.
From the L2TP subsystem's perspective it is a bad idea, since by
encouraging fastd to use the L2TP code, we end up hampering future L2TP
development in order to support a project which doesn't actually use
the L2TP protocol at all.
As explained above, this only concerns fastd's handshake format. As long as
no new L2TP version accepts 0 as its "Version" field and such packets
continue to passed to userspace even without the T flag, fastd would be
fine.
quoted
In the hope of being more constructive -- have you considered whether
tc and/or ebpf could be used for fastd? As more generic mechanisms I
think you might get on better with these than trying to twist the L2TP
code's arm :-)
(e)BPF might actually be an option. I will look into this.
quoted
quoted
quoted
quoted
After the mentioned change, this approach would lead to significant log
spam, as the previously hidden warnings are now shown by default. Not
even setting the T flag on the custom control packets is sufficient to
surpress these warnings, as packet length and L2TP version are checked
before the T flag.
Possibly we could sidestep some of these warnings by moving the T flag
check further up in the function.
The code would need to pull the first byte of the header, check the type
bit, and skip further processing if the bit was set. Otherwise go on to
pull the rest of the header.
I think I'd prefer this approach assuming the warnings are not
triggered by valid L2TP messages. >>
This will not be sufficient for my usecase: To stay compatible with older
versions of fastd, I can't set the T flag in the first packet of the
handshake, as it won't be known whether the peer has a new enough fastd
version to understand packets that have this bit set. Luckily, the second
handshake byte is always 0 in fastd's protocol, so these packets fail the
tunnel version check and are passed to userspace regardless.
I'm aware that this usecase is far outside of the original intentions of the
code and can only be described as a hack, but I still consider this a
regression in the kernel, as it was working fine in the past, without
visible warnings.
I'm sorry, but for the reasons stated above I disagree about it being
a regression.
Hmm, is it common for protocol implementations in the kernel to warn about
invalid packets they receive? While L2TP uses connected sockets and thus
usually no unrelated packets end up in the socket, a simple UDP port scan
originating from the configured remote address/port will trigger the "short
packet" warning now (nmap uses a zero-length payload for UDP scans by
default). Log spam caused by a malicous party might also be a concern.
It's not unheard of, but it's not especially common either. You can
see other pr_warn_ratelimit in the recv path in net/, but not very
many. I note that pr_debug_ratelimit is much rarer.
I appreciate the argument about log spam -- I'm not really wedded to
these log messages per se, but it seemed worthwhile exploring why you
were tripping over them.
If fastd's data packet headers end up being L2TP headers when using
the L2TP data path, I don't think there's too much of an issue to be
honest.
Reduce all warnings debug level when packets are passed to userspace.
Fixes: 5ee759cda51b ("l2tp: use standard API for warning log messages")
Signed-off-by: Matthias Schiffer <redacted>
quoted
quoted
---
I'm unsure what to do about the pr_warn_ratelimited() in
l2tp_recv_common(). It feels wrong to me that an incoming network packet
can trigger a kernel message above debug level at all, so maybe they
should be downgraded as well? I believe the only reason these were ever
warnings is that they were not shown by default.
net/l2tp/l2tp_core.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
@@ -809,8 +809,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Short packet? */if(!pskb_may_pull(skb,L2TP_HDR_SIZE_MAX)){-pr_warn_ratelimited("%s: recv short packet (len=%d)\n",-tunnel->name,skb->len);+pr_debug_ratelimited("%s: recv short packet (len=%d)\n",+tunnel->name,skb->len);gotoerror;}
@@ -824,8 +824,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)/* Check protocol version */version=hdrflags&L2TP_HDR_VER_MASK;if(version!=tunnel->version){-pr_warn_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",-tunnel->name,version,tunnel->version);+pr_debug_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",+tunnel->name,version,tunnel->version);gotoerror;}
@@ -863,8 +863,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)l2tp_session_dec_refcount(session);/* Not found? Pass to userspace to deal with */-pr_warn_ratelimited("%s: no session found (%u/%u). Passing up.\n",-tunnel->name,tunnel_id,session_id);+pr_debug_ratelimited("%s: no session found (%u/%u). Passing up.\n",+tunnel->name,tunnel_id,session_id);gotoerror;}