Subject: Re: [RFC PATCH] bpf: handle 32-bit zext during constant blinding

From: Jiong Wang <[email-masked]>
Date: Wed, 21 Aug 2019 10:05:53 +0100

Naveen N. Rao writes:

> Naveen N. Rao wrote:
>> Since BPF constant blinding is performed after the verifier pass, there
>> are certain ALU32 instructions inserted which don't have a corresponding
>> zext instruction inserted after. 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 <[email-masked]>
>> Signed-off-by: Naveen N. Rao <[email-masked]>
>> ---
>> This approach (the location where zext is being introduced below, in 
>> particular) works for powerpc, but I am not entirely sure if this is 
>> sufficient for other architectures as well. This is broken on v5.3-rc4.
>
> Alexie, Daniel, Jiong,
> Any feedback on this?

The fix on BPF_LD | BPF_IMM | BPF_DW looks correct to me, but the two other
places looks to me is unnecessary, as those destinations are exposed to
external and if they are used as 64-bit then there will be zext inserted
for them.

Have you verified removing those two fixes will still cause the bug?

Regards,
Jiong

> - Naveen