netfilter: nft_exthdr: avoid 60 bytes stack buffer in TCP option mangling

nft_exthdr_tcp_set_eval() and nft_exthdr_tcp_strip_eval() fetched the
TCP header and options through nft_tcp_header_pointer(), which calls
skb_header_pointer() a second time for the full tcphdr_len into a 60
bytes stack buffer. Since commit 28427f368f0e ("netfilter: nft_exthdr:
Fix non-linear header modification") both functions then call
skb_ensure_writable() and re-read the header from skb->data, so the
buffer was never used.

Only the 20 bytes fixed header is needed to learn tcphdr_len, so fetch
just that, validate the length as before, and let skb_ensure_writable()
pull the rest. This also drops the 60 bytes stack buffer from both
functions.

Comparison of performance in mpps:

  op       linear/non-linear     before    after    delta
==================================================================
   set,         non-linear       4.193     4.348    +3.7% mpps
   strip,       non-linear       4.184     4.434    +6.0% mpps
   set,         linear           5.302     5.427    +2.4% mpps
   strip,       linear           5.452     5.527    +1.4% mpss

Comparison of performance in ns/expression:

  op       linear/non-linear     before    after    delta
==================================================================
   set,         non-linear       34.6      25.3     -9.4  ns
   strip,       non-linear       35.2      20.8     -14.3 ns
   set,         linear           23.6      19.7     -3.9  ns
   strip,       linear           22.9      19.8     -3.1  ns

./scripts/bloat-o-meter exthdr_before.o exthdr_after.o
add/remove: 0/1 grow/shrink: 3/0 up/down: 233/-207 (26)
Function                                     old     new   delta
nft_exthdr_tcp_eval                          792     946    +154
nft_exthdr_tcp_strip_eval                    825     891     +66
nft_exthdr_tcp_set_eval                      602     615     +13
nft_tcp_header_pointer.part.constprop        207       -    -207
Total: Before=6527, After=6553, chg +0.40%

This also allow the compiler to inline nft_tcp_header_pointer inside
nft_exthdr_tcp_eval() which could yield some minimal performance gain.

This patch also updates the logic to drop malformed packets, before this
patch NFT_BREAK was used as a verdict instead.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

0 files changed, 0 insertions, 0 deletions