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

Le 31/03/2021 à 12:37, Michael Ellerman a écrit :

Jordan Niethe [off-list ref] writes:

quoted

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 <redacted>
---
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.

I was about to do the same comment, but ....

CONFIG_STRICT_MODULE_RWX is not CONFIG_ARCH_HAS_SET_MEMORY

quoted

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.

Same

quoted

@@ -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?) ?

cheers

quoted

  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