Re: [PATCH bpf-next 1/2] bpf: Add bpf_lsm_set_bprm_opts helper

[Daniel Borkmann <daniel@iogearbox.net>]

On 11/16/20 3:01 PM, KP Singh wrote:
> From: KP Singh <kpsingh@google.com>
> 
> The helper allows modification of certain bits on the linux_binprm
> struct starting with the secureexec bit which can be updated using the
> BPF_LSM_F_BPRM_SECUREEXEC flag.
> 
> secureexec can be set by the LSM for privilege gaining executions to set
> the AT_SECURE auxv for glibc.  When set, the dynamic linker disables the
> use of certain environment variables (like LD_PRELOAD).
> 
> Signed-off-by: KP Singh <kpsingh@google.com>
[...]
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> @@ -4119,6 +4128,11 @@ enum bpf_lwt_encap_mode {
>   	BPF_LWT_ENCAP_IP,
>   };
>   
> +/* Flags for LSM helpers */
> +enum {
> +	BPF_LSM_F_BPRM_SECUREEXEC	= (1ULL << 0),
> +};
> +
>   #define __bpf_md_ptr(type, name)	\
>   union {					\
>   	type name;			\
> diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> index 553107f4706a..4d04fc490a14 100644
> --- a/kernel/bpf/bpf_lsm.c
> +++ b/kernel/bpf/bpf_lsm.c
> @@ -7,6 +7,7 @@
>   #include <linux/filter.h>
>   #include <linux/bpf.h>
>   #include <linux/btf.h>
> +#include <linux/binfmts.h>
>   #include <linux/lsm_hooks.h>
>   #include <linux/bpf_lsm.h>
>   #include <linux/kallsyms.h>
> @@ -51,6 +52,23 @@ int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
>   	return 0;
>   }
>   
> +BPF_CALL_2(bpf_lsm_set_bprm_opts, struct linux_binprm *, bprm, u64, flags)
> +{

This should also reject invalid flags. I'd rather change this helper from RET_VOID
to RET_INTEGER and throw -EINVAL for everything other than BPF_LSM_F_BPRM_SECUREEXEC
passed in here including zero so it can be extended in future.

> +	bprm->secureexec = (flags & BPF_LSM_F_BPRM_SECUREEXEC);
> +	return 0;
> +}
> +
> +BTF_ID_LIST_SINGLE(bpf_lsm_set_bprm_opts_btf_ids, struct, linux_binprm)
> +
> +const static struct bpf_func_proto bpf_lsm_set_bprm_opts_proto = {
> +	.func		= bpf_lsm_set_bprm_opts,
> +	.gpl_only	= false,
> +	.ret_type	= RET_VOID,
> +	.arg1_type	= ARG_PTR_TO_BTF_ID,
> +	.arg1_btf_id	= &bpf_lsm_set_bprm_opts_btf_ids[0],
> +	.arg2_type	= ARG_ANYTHING,
> +};
> +