Re: [PATCH] bpf: validate bpf_func when BPF_JIT is enabled

[Yonghong Song <yhs@fb.com>]

On 9/9/19 11:32 PM, Sami Tolvanen wrote:
> With CONFIG_BPF_JIT, the kernel makes indirect calls to dynamically
> generated code. This change adds basic sanity checking to ensure
> we are jumping to a valid location, which narrows down the attack
> surface on the stored pointer. This also prepares the code for future
> Control-Flow Integrity (CFI) checking, which adds indirect call
> validation to call targets that can be determined at compile-time, but
> cannot validate calls to jited functions.
> 
> In addition, this change adds a weak arch_bpf_jit_check_func function,
> which architectures that implement BPF JIT can override to perform
> additional validation, such as verifying that the pointer points to
> the correct memory region.

You did not mention BPF_BINARY_HEADER_MAGIC and added member
of `magic` in bpf_binary_header. Could you add some details
on what is the purpose for this `magic` member?

> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
>   include/linux/filter.h | 26 ++++++++++++++++++++++++--
>   kernel/bpf/core.c      | 25 +++++++++++++++++++++++++
>   2 files changed, 49 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 92c6e31fb008..abfb0e1b21a8 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -511,7 +511,10 @@ struct sock_fprog_kern {
>   	struct sock_filter	*filter;
>   };
>   
> +#define BPF_BINARY_HEADER_MAGIC	0x05de0e82
> +
>   struct bpf_binary_header {
> +	u32 magic;
>   	u32 pages;
>   	/* Some arches need word alignment for their instructions */
>   	u8 image[] __aligned(4);
> @@ -553,20 +556,39 @@ struct sk_filter {
>   
>   DECLARE_STATIC_KEY_FALSE(bpf_stats_enabled_key);
>   
> +#ifdef CONFIG_BPF_JIT
> +/*
> + * With JIT, the kernel makes an indirect call to dynamically generated
> + * code. Use bpf_call_func to perform additional validation of the call
> + * target to narrow down attack surface. Architectures implementing BPF
> + * JIT can override arch_bpf_jit_check_func for arch-specific checking.
> + */
> +extern unsigned int bpf_call_func(const struct bpf_prog *prog,
> +				  const void *ctx);
> +
> +extern bool arch_bpf_jit_check_func(const struct bpf_prog *prog);
> +#else
> +static inline unsigned int bpf_call_func(const struct bpf_prog *prog,
> +					 const void *ctx)
> +{
> +	return prog->bpf_func(ctx, prog->insnsi);
> +}
> +#endif
> +
>   #define BPF_PROG_RUN(prog, ctx)	({				\
>   	u32 ret;						\
>   	cant_sleep();						\
>   	if (static_branch_unlikely(&bpf_stats_enabled_key)) {	\
>   		struct bpf_prog_stats *stats;			\
>   		u64 start = sched_clock();			\
> -		ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi);	\
> +		ret = bpf_call_func(prog, ctx);			\
>   		stats = this_cpu_ptr(prog->aux->stats);		\
>   		u64_stats_update_begin(&stats->syncp);		\
>   		stats->cnt++;					\
>   		stats->nsecs += sched_clock() - start;		\
>   		u64_stats_update_end(&stats->syncp);		\
>   	} else {						\
> -		ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi);	\
> +		ret = bpf_call_func(prog, ctx);			\
>   	}							\
>   	ret; })
>   
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 66088a9e9b9e..7aad58f67105 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -792,6 +792,30 @@ void __weak bpf_jit_free_exec(void *addr)
>   	module_memfree(addr);
>   }
>   
> +#ifdef CONFIG_BPF_JIT
> +bool __weak arch_bpf_jit_check_func(const struct bpf_prog *prog)
> +{
> +	return true;
> +}
> +
> +unsigned int bpf_call_func(const struct bpf_prog *prog, const void *ctx)
> +{
> +	const struct bpf_binary_header *hdr = bpf_jit_binary_hdr(prog);
> +
> +	if (!IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) && !prog->jited)
> +		return prog->bpf_func(ctx, prog->insnsi);
> +
> +	if (unlikely(hdr->magic != BPF_BINARY_HEADER_MAGIC ||
> +		     !arch_bpf_jit_check_func(prog))) {
> +		WARN(1, "attempt to jump to an invalid address");
> +		return 0;
> +	}
> +
> +	return prog->bpf_func(ctx, prog->insnsi);
> +}

The above can be rewritten as
	if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) || prog->jited ||
	    hdr->magic != BPF_BINARY_HEADER_MAGIC ||
	    !arch_bpf_jit_check_func(prog))) {
		WARN(1, "attempt to jump to an invalid address");
		return 0;
	}

	return prog->bpf_func(ctx, prog->insnsi);

BPF_PROG_RUN() will be called during xdp fast path.
Have you measured how much slowdown the above change could
cost for the performance?

> +EXPORT_SYMBOL_GPL(bpf_call_func);
> +#endif
> +
>   struct bpf_binary_header *
>   bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>   		     unsigned int alignment,
> @@ -818,6 +842,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>   	/* Fill space with illegal/arch-dep instructions. */
>   	bpf_fill_ill_insns(hdr, size);
>   
> +	hdr->magic = BPF_BINARY_HEADER_MAGIC;
>   	hdr->pages = pages;
>   	hole = min_t(unsigned int, size - (proglen + sizeof(*hdr)),
>   		     PAGE_SIZE - sizeof(*hdr));
>