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

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Message-ID: <20190425204601.wbymvydzahiti4id@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 25 Apr 2019 13:46:03 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Paul Chaignon <paul.chaignon@...nge.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Xiao Han <xiao.han@...nge.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf v3 0/2] bpf: mark registers in all frames after
 pkt/null checks

On Wed, Apr 24, 2019 at 09:49:58PM +0200, 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.

Applied to bpf tree, thanks!