Re: [PATCH v9 3/8] powerpc/kprobes: Mark newly allocated probes as RO

[Jordan Niethe <jniethe5@gmail.com>]

On Wed, Mar 17, 2021 at 5:12 PM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 16/03/2021 à 04:17, Jordan Niethe a écrit :
> > From: Russell Currey <ruscur@russell.cc>
> >
> > With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
> > W+X page at boot by default.  This can be tested with
> > CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
> > kernel log during boot.
> >
> > Add an arch specific insn page allocator which returns RO pages if
> > STRICT_KERNEL_RWX is enabled. This page is only written to with
> > patch_instruction() which is able to write RO pages.
> >
>
> Did you investigate BPF ? The problematic looks more or less similar to kprobe:
>
> bpf_jit_compile() in arch/powerpc/net/bpf_jit_comp.c calls module_alloc(), which provides it with
> PAGE_KERNEL_TEXT memory, ie RWX. That function is only used on PPC32 which still has Classic BPF,
> and this is about to go away with future series
> https://patchwork.ozlabs.org/project/linuxppc-dev/cover/cover.1608112796.git.christophe.leroy@csgroup.eu/
>
> PPC64 has Extended BPF instead, and PPC32 will it the future too.
> bpf_int_jit_compile() in arch/powerpc/net/bpf_jit_comp64.c calls bpf_jit_binary_alloc() which uses
> bpf_jit_alloc_exec().
>
> bpf_jit_alloc_exec() is a weak function that should be redefined for powerpc I think, more or less
> like alloc_insn_page() for kprobes.
Thanks, that is a good point. I will handle bpf with the next revision.
>
> Christophe