Re: [PATCH v14 6/9] powerpc/bpf: Write protect JIT code

[Jordan Niethe <jniethe5@gmail.com>]

On Mon, May 17, 2021 at 4:40 PM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 17/05/2021 à 05:28, Jordan Niethe a écrit :
> > Add the necessary call to bpf_jit_binary_lock_ro() to remove write and
> > add exec permissions to the JIT image after it has finished being
> > written.
> >
> > Without CONFIG_STRICT_MODULE_RWX the image will be writable and
> > executable until the call to bpf_jit_binary_lock_ro().
>
> And _with_ CONFIG_STRICT_MODULE_RWX what will happen ? It will be _writable_ but not _executable_ ?
That's right.
With CONFIG_STRICT_MODULE_RWX the image will initially be PAGE_KERNEL
from bpf_jit_alloc_exec() calling module_alloc(). So not executable.
bpf_jit_binary_lock_ro() will then remove write and add executable.

Without CONFIG_STRICT_MODULE_RWX the image will initially be
PAGE_KERNEL_EXEC from module_alloc().
bpf_jit_binary_lock_ro() will remove write, but until that point it
will have been write + exec.
>
> >
> > Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> > Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
> > ---
> > v10: New to series
> > v11: Remove CONFIG_STRICT_MODULE_RWX conditional
> > ---
> >   arch/powerpc/net/bpf_jit_comp.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> > index 6c8c268e4fe8..53aefee3fe70 100644
> > --- a/arch/powerpc/net/bpf_jit_comp.c
> > +++ b/arch/powerpc/net/bpf_jit_comp.c
> > @@ -237,6 +237,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> >       fp->jited_len = alloclen;
> >
> >       bpf_flush_icache(bpf_hdr, (u8 *)bpf_hdr + (bpf_hdr->pages * PAGE_SIZE));
> > +     bpf_jit_binary_lock_ro(bpf_hdr);
> >       if (!fp->is_func || extra_pass) {
> >               bpf_prog_fill_jited_linfo(fp, addrs);
> >   out_addrs:
> >