netdev - Re: [PATCH bpf v2 0/2] bpf: mark registers as safe or unknown in all frames

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Message-ID: <58187776-5cce-6a8b-cff1-3b8872c6df8f@fb.com>
Date:   Mon, 22 Apr 2019 22:07:46 +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 v2 0/2] bpf: mark registers as safe or unknown in all
 frames



On 4/22/19 11:34 AM, 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.  This first patch also marks registers in
> previous frames.
> 
> A good reproducer 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 v2:
>      - Fix example codes in commit message.
> 
> Paul Chaignon (2):
>    bpf: mark registers as safe or unknown in all frames
>    selftests/bpf: test case for pointer null check in subprog
> 
>   kernel/bpf/verifier.c                        |  6 ++---
>   tools/testing/selftests/bpf/verifier/calls.c | 25 ++++++++++++++++++++
>   2 files changed, 28 insertions(+), 3 deletions(-)

For the series:
   Acked-by: Yonghong Song <yhs@...com>