[PATCH v10 05/10] powerpc/bpf: Write protect JIT code

[Jordan Niethe]

On Wed, Mar 31, 2021 at 9:37 PM Michael Ellerman <mpe at ellerman.id.au> wrote:
>
> Jordan Niethe <jniethe5 at gmail.com> writes:
>
> > Once CONFIG_STRICT_MODULE_RWX is enabled there will be no need to
> > override bpf_jit_free() because it is now possible to set images
> > read-only. So use the default implementation.
> >
> > Also add the necessary call to bpf_jit_binary_lock_ro() which will
> > remove write protection and add exec protection to the JIT image after
> > it has finished being written.
> >
> > Signed-off-by: Jordan Niethe <jniethe5 at gmail.com>
> > ---
> > v10: New to series
> > ---
> >  arch/powerpc/net/bpf_jit_comp.c   | 5 ++++-
> >  arch/powerpc/net/bpf_jit_comp64.c | 4 ++++
> >  2 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> > index e809cb5a1631..8015e4a7d2d4 100644
> > --- a/arch/powerpc/net/bpf_jit_comp.c
> > +++ b/arch/powerpc/net/bpf_jit_comp.c
> > @@ -659,12 +659,15 @@ void bpf_jit_compile(struct bpf_prog *fp)
> >               bpf_jit_dump(flen, proglen, pass, code_base);
> >
> >       bpf_flush_icache(code_base, code_base + (proglen/4));
> > -
> >  #ifdef CONFIG_PPC64
> >       /* Function descriptor nastiness: Address + TOC */
> >       ((u64 *)image)[0] = (u64)code_base;
> >       ((u64 *)image)[1] = local_paca->kernel_toc;
> >  #endif
> > +     if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX)) {
> > +             set_memory_ro((unsigned long)image, alloclen >> PAGE_SHIFT);
> > +             set_memory_x((unsigned long)image, alloclen >> PAGE_SHIFT);
> > +     }
>
> You don't need to check the ifdef in a caller, there are stubs that
> compile to nothing when CONFIG_ARCH_HAS_SET_MEMORY=n.
As Christophe pointed out we could have !CONFIG_STRICT_MODULE_RWX and
CONFIG_ARCH_HAS_SET_MEMORY which would then be wrong here.
Probably we could make CONFIG_ARCH_HAS_SET_MEMORY depend on
CONFIG_STRICT_MODULE_RWX?
>
> > diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> > index aaf1a887f653..1484ad588685 100644
> > --- a/arch/powerpc/net/bpf_jit_comp64.c
> > +++ b/arch/powerpc/net/bpf_jit_comp64.c
> > @@ -1240,6 +1240,8 @@ 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));
> > +     if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
> > +             bpf_jit_binary_lock_ro(bpf_hdr);
>
> Do we need the ifdef here either? Looks like it should be safe to call
> due to the stubs.
>
> > @@ -1262,6 +1264,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> >  }
> >
> >  /* Overriding bpf_jit_free() as we don't set images read-only. */
> > +#ifndef CONFIG_STRICT_MODULE_RWX
>
> Did you test without this and notice something broken?
>
> Looking at the generic version I can't tell why we need to override
> this. Maybe we don't (anymore?) ?
Yeah we don't.
>
> cheers
>
> >  void bpf_jit_free(struct bpf_prog *fp)
> >  {
> >       unsigned long addr = (unsigned long)fp->bpf_func & PAGE_MASK;
> > @@ -1272,3 +1275,4 @@ void bpf_jit_free(struct bpf_prog *fp)
> >
> >       bpf_prog_unlock_free(fp);
> >  }
> > +#endif
> > --
> > 2.25.1