linux-kernel - Re: [PATCH] bpf: handle 32-bit zext during constant blinding

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Message-Id: <1566802541.7onbueyw0d.naveen@linux.ibm.com>
Date:   Mon, 26 Aug 2019 12:29:05 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Jiong Wang <jiong.wang@...ronome.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>, netdev@...r.kernel.org
Subject: Re: [PATCH] bpf: handle 32-bit zext during constant blinding

Jiong Wang wrote:
> 
> Naveen N. Rao writes:
> 
>> Since BPF constant blinding is performed after the verifier pass, the
>> ALU32 instructions inserted for doubleword immediate loads don't have a
>> corresponding zext instruction. This is causing a kernel oops on powerpc
>> and can be reproduced by running 'test_cgroup_storage' with
>> bpf_jit_harden=2.
>>
>> Fix this by emitting BPF_ZEXT during constant blinding if
>> prog->aux->verifier_zext is set.
>>
>> Fixes: a4b1d3c1ddf6cb ("bpf: verifier: insert zero extension according to analysis result")
>> Reported-by: Michael Ellerman <mpe@...erman.id.au>
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> 
> Thanks for the fix.
> 
> Reviewed-by: Jiong Wang <jiong.wang@...ronome.com>
> 
> Just two other comments during review in case I am wrong on somewhere.
> 
>   - Use verifier_zext instead of bpf_jit_needs_zext() seems better, even
>     though the latter could avoid extending function argument.
> 
>     Because JIT back-ends look at verifier_zext, true means zext inserted
>     by verifier so JITs won't do the code-gen.
> 
>     Use verifier_zext is sort of keeping JIT blinding the same behaviour
>     has verifier even though blinding doesn't belong to verifier, but for
>     such insn patching, it could be seen as a extension of verifier,
>     therefore use verifier_zext seems better than bpf_jit_needs_zext() to
>     me.
>    
>   - JIT blinding is also escaping the HI32 randomization which happens
>     inside verifier, otherwise x86-64 regression should have caught this issue.

Jiong,
Thanks for the review.

Alexei, Daniel,
Can you please pick this up for v5.3. This is a regression and is 
causing a crash on powerpc.


- Naveen