Re: [PATCH bpf-next v3 4/7] bpf: Attachment verification for BPF_MODIFY_RETURN

[Casey Schaufler <casey@schaufler-ca.com>]

On 3/4/2020 7:47 AM, KP Singh wrote:
> From: KP Singh <kpsingh@google.com>
>
> - Allow BPF_MODIFY_RETURN attachment only to functions that are:
>
>     * Whitelisted for error injection by checking
>       within_error_injection_list. Similar discussions happened for the
>       bpf_override_return helper.
>
>     * security hooks, this is expected to be cleaned up with the LSM
>       changes after the KRSI patches introduce the LSM_HOOK macro:
>
>         https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/

Be careful with this forward reference. Changes suggested to LSM_HOOK
macros remain contentious.

>
> - The attachment is currently limited to functions that return an int.
>   This can be extended later other types (e.g. PTR).
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> Acked-by: Andrii Nakryiko <andriin@fb.com>
> ---
>  kernel/bpf/btf.c      | 28 ++++++++++++++++++++--------
>  kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 30841fb8b3c0..50080add2ab9 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3710,14 +3710,26 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>  		nr_args--;
>  	}
>  
> -	if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||
> -	     prog->expected_attach_type == BPF_MODIFY_RETURN) &&
> -	    arg == nr_args) {
> -		if (!t)
> -			/* Default prog with 5 args. 6th arg is retval. */
> -			return true;
> -		/* function return type */
> -		t = btf_type_by_id(btf, t->type);
> +	if (arg == nr_args) {
> +		if (prog->expected_attach_type == BPF_TRACE_FEXIT) {
> +			if (!t)
> +				return true;
> +			t = btf_type_by_id(btf, t->type);
> +		} else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
> +			/* For now the BPF_MODIFY_RETURN can only be attached to
> +			 * functions that return an int.
> +			 */
> +			if (!t)
> +				return false;
> +
> +			t = btf_type_skip_modifiers(btf, t->type, NULL);
> +			if (!btf_type_is_int(t)) {
> +				bpf_log(log,
> +					"ret type %s not allowed for fmod_ret\n",
> +					btf_kind_str[BTF_INFO_KIND(t->info)]);
> +				return false;
> +			}
> +		}
>  	} else if (arg >= nr_args) {
>  		bpf_log(log, "func '%s' doesn't have %d-th argument\n",
>  			tname, arg + 1);
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 2460c8e6b5be..ae32517d4ccd 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -19,6 +19,7 @@
>  #include <linux/sort.h>
>  #include <linux/perf_event.h>
>  #include <linux/ctype.h>
> +#include <linux/error-injection.h>
>  
>  #include "disasm.h"
>  
> @@ -9800,6 +9801,33 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
>  
>  	return 0;
>  }
> +#define SECURITY_PREFIX "security_"
> +
> +static int check_attach_modify_return(struct bpf_verifier_env *env)
> +{
> +	struct bpf_prog *prog = env->prog;
> +	unsigned long addr = (unsigned long) prog->aux->trampoline->func.addr;
> +
> +	if (within_error_injection_list(addr))
> +		return 0;
> +
> +	/* This is expected to be cleaned up in the future with the KRSI effort
> +	 * introducing the LSM_HOOK macro for cleaning up lsm_hooks.h.
> +	 */
> +	if (!strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
> +		     sizeof(SECURITY_PREFIX) - 1)) {
> +
> +		if (!capable(CAP_MAC_ADMIN))
> +			return -EPERM;
> +
> +		return 0;
> +	}
> +
> +	verbose(env, "fmod_ret attach_btf_id %u (%s) is not modifiable\n",
> +		prog->aux->attach_btf_id, prog->aux->attach_func_name);
> +
> +	return -EINVAL;
> +}
>  
>  static int check_attach_btf_id(struct bpf_verifier_env *env)
>  {
> @@ -10000,6 +10028,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>  		}
>  		tr->func.addr = (void *)addr;
>  		prog->aux->trampoline = tr;
> +
> +		if (prog->expected_attach_type == BPF_MODIFY_RETURN)
> +			ret = check_attach_modify_return(env);
>  out:
>  		mutex_unlock(&tr->mutex);
>  		if (ret)