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Message-ID: <793acf23-b263-6ae5-2206-18fcdfa991eb@iogearbox.net>
Date: Mon, 16 Nov 2020 16:11:49 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: KP Singh <kpsingh@...omium.org>, linux-kernel@...r.kernel.org,
bpf@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Paul Turner <pjt@...gle.com>,
Pauline Middelink <middelin@...gle.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: Add bpf_lsm_set_bprm_opts helper
On 11/16/20 3:01 PM, KP Singh wrote:
> From: KP Singh <kpsingh@...gle.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@...gle.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,
> +};
> +
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