Re: Regression fix for bpf in v5.3 (was Re: [RFC PATCH] bpf: handle 32-bit zext during constant blinding)

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Jiong Wang wrote:
> 
> Michael Ellerman writes:
> 
>> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> writes:
>>> 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 <mpe@ellerman.id.au>
>>> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>>> ---
>>> 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.
>>
>> Any comment on this?
> 
> Have commented on https://marc.info/?l=linux-netdev&m=156637836024743&w=2
> 
> The fix looks correct to me on "BPF_LD | BPF_IMM | BPF_DW", but looks
> unnecessary on two other places. It would be great if you or Naveen could
> confirm it.

Jiong,
Thanks for the review. I can now see why the other two changes are not 
necessary. I will post a follow-on patch.

Thanks!
- Naveen