netdev - [PATCH bpf v3 0/2] bpf: mark registers in all frames after pkt/null checks

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Date:   Wed, 24 Apr 2019 21:49:58 +0200
From:   Paul Chaignon <paul.chaignon@...nge.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     Xiao Han <xiao.han@...nge.com>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: [PATCH bpf v3 0/2] bpf: mark registers in all frames after pkt/null
 checks

In case of a null check on a pointer inside a subprog, we should mark all
registers with this pointer as either safe or unknown, in both the current
and previous frames.  Currently, only spilled registers and registers in
the current frame are marked.  Packet bound checks in subprogs have the
same issue.  The first patch fixes it to mark registers in previous frames
as well.

A good reproducer for null checks looks as follow:

1: ptr = bpf_map_lookup_elem(map, &key);
2: ret = subprog(ptr) {
3:   return ptr != NULL;
4: }
5: if (ret)
6:   value = *ptr;

With the above, the verifier will complain on line 6 because it sees ptr
as map_value_or_null despite the null check in subprog 1.  The second
patch implements the above as a new test case.

Note that this patch fixes another resulting bug when using
bpf_sk_release():

1: sk = bpf_sk_lookup_tcp(...);
2: subprog(sk) {
3:   if (sk)
4:     bpf_sk_release(sk);
5: }
6: if (!sk)
7:   return 0;
8: return 1;

In the above, mark_ptr_or_null_regs will warn on line 6 because it will
try to free the reference state, even though it was already freed on
line 3.

Changelogs:
  Changes in v3:
    - Fix same issue in find_good_pkt_pointers().
    - Add test case for find_good_pkt_pointers() issue.
    - Change title to account for the above change. Old title was
      "bpf: mark registers as safe or unknown in all frames".
    - Refactor find_good_pkt_pointers and mark_ptr_or_null_regs.
    - I did not keep Yonghong's ack because of the above changes.
  Changes in v2:
    - Fix example codes in commit message.

Paul Chaignon (2):
  bpf: mark registers in all frames after pkt/null checks
  selftests/bpf: test cases for pkt/null checks in subprogs

 kernel/bpf/verifier.c                         | 76 +++++++++++--------
 tools/testing/selftests/bpf/verifier/calls.c  | 25 ++++++
 .../bpf/verifier/direct_packet_access.c       | 22 ++++++
 3 files changed, 93 insertions(+), 30 deletions(-)

-- 
2.17.1