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

On Mon, May 17, 2021 at 4:40 PM Christophe Leroy
[off-list ref] wrote:

Le 17/05/2021 à 05:28, Jordan Niethe a écrit :

quoted

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.

quoted

Reviewed-by: Christophe Leroy <redacted>
Signed-off-by: Jordan Niethe <redacted>
---
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: