Re: [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and execution

Yonghong Song
2020-03-23T19:17:01Z

On 3/23/20 9:44 AM, KP Singh wrote:
> From: KP Singh <kpsingh@google.com>
>
> JITed BPF programs are dynamically attached to the LSM hooks
> using BPF trampolines. The trampoline prologue generates code to handle
> conversion of the signature of the hook to the appropriate BPF context.
>
> The allocated trampoline programs are attached to the nop functions
> initialized as LSM hooks.
>
> BPF_PROG_TYPE_LSM programs must have a GPL compatible license and
> and need CAP_SYS_ADMIN (required for loading eBPF programs).
>
> Upon attachment:
>
> * A BPF fexit trampoline is used for LSM hooks with a void return type.
> * A BPF fmod_ret trampoline is used for LSM hooks which return an
> int. The attached programs can override the return value of the
> bpf LSM hook to indicate a MAC Policy decision.
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> Reviewed-by: Brendan Jackman <jackmanb@google.com>
> Reviewed-by: Florent Revest <revest@google.com>
> ---
> include/linux/bpf.h | 4 ++++
> include/linux/bpf_lsm.h | 11 +++++++++++
> kernel/bpf/bpf_lsm.c | 29 +++++++++++++++++++++++++++++
> kernel/bpf/btf.c | 9 ++++++++-
> kernel/bpf/syscall.c | 26 ++++++++++++++++++++++----
> kernel/bpf/trampoline.c | 17 +++++++++++++----
> kernel/bpf/verifier.c | 19 +++++++++++++++----
> 7 files changed, 102 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index af81ec7b783c..adf2e5a6de4b 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -433,6 +433,10 @@ struct btf_func_model {
> * programs only. Should not be used with normal calls and indirect calls.
> */
> #define BPF_TRAMP_F_SKIP_FRAME BIT(2)
> +/* Override the return value of the original function. This flag only makes
> + * sense for fexit trampolines.
> + */
> +#define BPF_TRAMP_F_OVERRIDE_RETURN BIT(3)

Whether the return value is overridable is determined by hook return
type as below. Do we still need this flag?

>
> /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
> * bytes on x86. Pick a number to fit into BPF_IMAGE_SIZE / 2 …