bpf: Do not change gso_size during bpf_skb_change_proto()

This is technically a backwards incompatible change in behaviour, but I'm
going to argue that it is very unlikely to break things, and likely to fix
*far* more then it breaks.

In no particular order, various reasons follow:

(a) I've long had a bug assigned to myself to debug a super rare kernel crash
on Android Pixel phones which can (per stacktrace) be traced back to BPF clat
IPv6 to IPv4 protocol conversion causing some sort of ugly failure much later
on during transmit deep in the GSO engine, AFAICT precisely because of this
change to gso_size, though I've never been able to manually reproduce it. I
believe it may be related to the particular network offload support of attached
USB ethernet dongle being used for tethering off of an IPv6-only cellular
connection. The reason might be we end up with more segments than max permitted,
or with a GSO packet with only one segment... (either way we break some
assumption and hit a BUG_ON)

(b) There is no check that the gso_size is > 20 when reducing it by 20, so we
might end up with a negative (or underflowing) gso_size or a gso_size of 0.
This can't possibly be good. Indeed this is probably somehow exploitable (or
at least can result in a kernel crash) by delivering crafted packets and perhaps
triggering an infinite loop or a divide by zero... As a reminder: gso_size (MSS)
is related to MTU, but not directly derived from it: gso_size/MSS may be
significantly smaller then one would get by deriving from local MTU. And on
some NICs (which do loose MTU checking on receive, it may even potentially be
larger, for example my work pc with 1500 MTU can receive 1520 byte frames [and
sometimes does due to bugs in a vendor plat46 implementation]). Indeed even just
going from 21 to 1 is potentially problematic because it increases the number
of segments by a factor of 21 (think DoS, or some other crash due to too many
segments).

(c) It's always safe to not increase the gso_size, because it doesn't result in
the max packet size increasing.  So the skb_increase_gso_size() call was always
unnecessary for correctness (and outright undesirable, see later). As such the
only part which is potentially dangerous (ie. could cause backwards compatibility
issues) is the removal of the skb_decrease_gso_size() call.

(d) If the packets are ultimately destined to the local device, then there is
absolutely no benefit to playing around with gso_size. It only matters if the
packets will egress the device. ie. we're either forwarding, or transmitting
from the device.

(e) This logic only triggers for packets which are GSO. It does not trigger for
skbs which are not GSO. It will not convert a non-GSO MTU sized packet into a
GSO packet (and you don't even know what the MTU is, so you can't even fix it).
As such your transmit path must *already* be able to handle an MTU 20 bytes
larger then your receive path (for IPv4 to IPv6 translation) - and indeed 28
bytes larger due to IPv4 fragments. Thus removing the skb_decrease_gso_size()
call doesn't actually increase the size of the packets your transmit side must
be able to handle. ie. to handle non-GSO max-MTU packets, the IPv4/IPv6 device/
route MTUs must already be set correctly. Since for example with an IPv4 egress
MTU of 1500, IPv4 to IPv6 translation will already build 1520 byte IPv6 frames,
so you need a 1520 byte device MTU. This means if your IPv6 device's egress
MTU is 1280, your IPv4 route must be 1260 (and actually 1252, because of the
need to handle fragments). This is to handle normal non-GSO packets. Thus the
reduction is simply not needed for GSO packets, because when they're correctly
built, they will already be the right size.

(f) TSO/GSO should be able to exactly undo GRO: the number of packets (TCP
segments) should not be modified, so that TCP's MSS counting works correctly
(this matters for congestion control). If protocol conversion changes the
gso_size, then the number of TCP segments may increase or decrease. Packet loss
after protocol conversion can result in partial loss of MSS segments that the
sender sent. How's the sending TCP stack going to react to receiving ACKs/SACKs
in the middle of the segments it sent?

(g) skb_{decrease,increase}_gso_size() are already no-ops for GSO_BY_FRAGS
case (besides triggering WARN_ON_ONCE). This means you already cannot guarantee
that gso_size (and thus resulting packet MTU) is changed. ie. you must assume
it won't be changed.

(h) changing gso_size is outright buggy for UDP GSO packets, where framing
matters (I believe that's also the case for SCTP, but it's already excluded
by [g]).  So the only remaining case is TCP, which also doesn't want it
(see [f]).

(i) see also the reasoning on the previous attempt at fixing this
(commit fa7b83bf3b) which shows that the current
behaviour causes TCP packet loss:

  In the forwarding path GRO -> BPF 6 to 4 -> GSO for TCP traffic, the
  coalesced packet payload can be > MSS, but < MSS + 20.

  bpf_skb_proto_6_to_4() will upgrade the MSS and it can be > the payload
  length. After then tcp_gso_segment checks for the payload length if it
  is <= MSS. The condition is causing the packet to be dropped.

  tcp_gso_segment():
    [...]
    mss = skb_shinfo(skb)->gso_size;
    if (unlikely(skb->len <= mss)) goto out;
    [...]

Thus changing the gso_size is simply a very bad idea. Increasing is unnecessary
and buggy, and decreasing can go negative.

Fixes: 6578171a7f ("bpf: add bpf_skb_change_proto helper")
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Dongseok Yi <dseok.yi@samsung.com>
Cc: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/bpf/CANP3RGfjLikQ6dg=YpBU0OeHvyv7JOki7CyOUS9modaXAi-9vQ@mail.gmail.com
Link: https://lore.kernel.org/bpf/20210617000953.2787453-2-zenczykowski@gmail.com

net/core/filter.c

@@ -3263,8 +3263,6 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
 			shinfo->gso_type |=  SKB_GSO_TCPV6;
 		}
 
-		/* Due to IPv6 header, MSS needs to be downgraded. */
-		skb_decrease_gso_size(shinfo, len_diff);
 		/* Header must be checked, and gso_segs recomputed. */
 		shinfo->gso_type |= SKB_GSO_DODGY;
 		shinfo->gso_segs = 0;
@@ -3304,8 +3302,6 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
 			shinfo->gso_type |=  SKB_GSO_TCPV4;
 		}
 
-		/* Due to IPv4 header, MSS can be upgraded. */
-		skb_increase_gso_size(shinfo, len_diff);
 		/* Header must be checked, and gso_segs recomputed. */
 		shinfo->gso_type |= SKB_GSO_DODGY;
 		shinfo->gso_segs = 0;