On Mon, Nov 16, 2020 at 11:25:35PM +0000, KP Singh wrote:
> From: KP Singh <kpsi...@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 <kpsi...@google.com>
> ---
> include/uapi/linux/bpf.h | 14 ++++++++++++++
> kernel/bpf/bpf_lsm.c | 27 +++++++++++++++++++++++++++
> scripts/bpf_helpers_doc.py | 2 ++
> tools/include/uapi/linux/bpf.h | 14 ++++++++++++++
> 4 files changed, 57 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 162999b12790..7f1b6ba8246c 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3787,6 +3787,14 @@ union bpf_attr {
> * *ARG_PTR_TO_BTF_ID* of type *task_struct*.
> * Return
> * Pointer to the current task.
> + *
> + * long bpf_lsm_set_bprm_opts(struct linux_binprm *bprm, u64 flags)
> + *
> + * Description
> + * Sets certain options on the *bprm*:
> + *
> + * **BPF_LSM_F_BPRM_SECUREEXEC** Set the secureexec bit
> + * which sets the **AT_SECURE** auxv for glibc.
The return value needs to be documented also.
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -3948,6 +3956,7 @@ union bpf_attr {
> FN(task_storage_get), \
> FN(task_storage_delete), \
> FN(get_current_task_btf), \
> + FN(lsm_set_bprm_opts), \
> /* */
>
> /* 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..31f85474a0ef 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 +52,30 @@ int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> return 0;
> }
>
> +/* Mask for all the currently supported BPRM option flags */
> +#define BPF_LSM_F_BRPM_OPTS_MASK 0x1ULL
If there is a need to have v3, it will be better to use
BPF_LSM_F_BPRM_SECUREEXEC instead of 0x1ULL.
> +
> +BPF_CALL_2(bpf_lsm_set_bprm_opts, struct linux_binprm *, bprm, u64, flags)
> +{
> +
> + if (flags & ~BPF_LSM_F_BRPM_OPTS_MASK)
> + return -EINVAL;
> +
> + bprm->secureexec = (flags & BPF_LSM_F_BPRM_SECUREEXEC);
The intention of this helper is to set "or clear" a bit?
It may be useful to clarify the "clear" part in the doc also.
> + return 0;
> +}
> +
