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

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Date:   Thu, 25 Apr 2019 15:27:00 +0000
From:   Yonghong Song <yhs@...com>
To:     Paul Chaignon <paul.chaignon@...nge.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
CC:     Xiao Han <xiao.han@...nge.com>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH bpf v3 0/2] bpf: mark registers in all frames after
 pkt/null checks



On 4/24/19 12:49 PM, Paul Chaignon wrote:
> 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(-)

Ack for the whole series.
Acked-by: Yonghong Song <yhs@...com>