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

[Naveen N. Rao]

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@xxxxxxxxxxxxxx>
> > Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
>
> Thanks for the fix.
>
> Reviewed-by: Jiong Wang <jiong.wang@xxxxxxxxxxxxx>
>
> 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