ofp-actions: Fix use-after-free while decoding RAW_ENCAP.

While decoding RAW_ENCAP action, decode_ed_prop() might re-allocate
ofpbuf if there is no enough space left.  However, function
'decode_NXAST_RAW_ENCAP' continues to use old pointer to 'encap'
structure leading to write-after-free and incorrect decoding.

  ==3549105==ERROR: AddressSanitizer: heap-use-after-free on address
  0x60600000011a at pc 0x0000005f6cc6 bp 0x7ffc3a2d4410 sp 0x7ffc3a2d4408
  WRITE of size 2 at 0x60600000011a thread T0
    #0 0x5f6cc5 in decode_NXAST_RAW_ENCAP lib/ofp-actions.c:4461:20
    williamtu#1 0x5f0551 in ofpact_decode ./lib/ofp-actions.inc2:4777:16
    williamtu#2 0x5ed17c in ofpacts_decode lib/ofp-actions.c:7752:21
    williamtu#3 0x5eba9a in ofpacts_pull_openflow_actions__ lib/ofp-actions.c:7791:13
    williamtu#4 0x5eb9fc in ofpacts_pull_openflow_actions lib/ofp-actions.c:7835:12
    williamtu#5 0x64bb8b in ofputil_decode_packet_out lib/ofp-packet.c:1113:17
    williamtu#6 0x65b6f4 in ofp_print_packet_out lib/ofp-print.c:148:13
    williamtu#7 0x659e3f in ofp_to_string__ lib/ofp-print.c:1029:16
    williamtu#8 0x659b24 in ofp_to_string lib/ofp-print.c:1244:21
    williamtu#9 0x65a28c in ofp_print lib/ofp-print.c:1288:28
    williamtu#10 0x540d11 in ofctl_ofp_parse utilities/ovs-ofctl.c:2814:9
    williamtu#11 0x564228 in ovs_cmdl_run_command__ lib/command-line.c:247:17
    williamtu#12 0x56408a in ovs_cmdl_run_command lib/command-line.c:278:5
    williamtu#13 0x5391ae in main utilities/ovs-ofctl.c:179:9
    williamtu#14 0x7f6911ce9081 in __libc_start_main (/lib64/libc.so.6+0x27081)
    williamtu#15 0x461fed in _start (utilities/ovs-ofctl+0x461fed)

Fix that by getting a new pointer before using.

Credit to OSS-Fuzz.

Fuzzer regression test will fail only with AddressSanitizer enabled.

Reported-at: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=27851
Fixes: f839892 ("OF support and translation of generic encap and decap")
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/ofp-actions.c

@@ -4431,6 +4431,7 @@ decode_NXAST_RAW_ENCAP(const struct nx_action_encap *nae,
 {
  struct ofpact_encap *encap;
  const struct ofp_ed_prop_header *ofp_prop;
+ const size_t encap_ofs = out->size;
  size_t props_len;
  uint16_t n_props = 0;
  int err;
@@ -4458,6 +4459,7 @@ decode_NXAST_RAW_ENCAP(const struct nx_action_encap *nae,
  }
  n_props++;
  }
+ encap = ofpbuf_at_assert(out, encap_ofs, sizeof *encap);
  encap->n_props = n_props;
  out->header = &encap->ofpact;
  ofpact_finish_ENCAP(out, &encap);

tests/automake.mk

@@ -134,7 +134,8 @@ FUZZ_REGRESSION_TESTS = \
  tests/fuzz-regression/ofp_print_fuzzer-5722747668791296 \
  tests/fuzz-regression/ofp_print_fuzzer-6285128790704128 \
  tests/fuzz-regression/ofp_print_fuzzer-6470117922701312 \
- tests/fuzz-regression/ofp_print_fuzzer-6502620041576448
+ tests/fuzz-regression/ofp_print_fuzzer-6502620041576448 \
+ tests/fuzz-regression/ofp_print_fuzzer-6540965472632832
 $(srcdir)/tests/fuzz-regression-list.at: tests/automake.mk
  $(AM_V_GEN)for name in $(FUZZ_REGRESSION_TESTS); do \
  basename=`echo $$name | sed 's,^.*/,,'`; \

tests/fuzz-regression-list.at

@@ -21,3 +21,4 @@ TEST_FUZZ_REGRESSION([ofp_print_fuzzer-5722747668791296])
 TEST_FUZZ_REGRESSION([ofp_print_fuzzer-6285128790704128])
 TEST_FUZZ_REGRESSION([ofp_print_fuzzer-6470117922701312])
 TEST_FUZZ_REGRESSION([ofp_print_fuzzer-6502620041576448])
+TEST_FUZZ_REGRESSION([ofp_print_fuzzer-6540965472632832])