On Tue, Nov 17, 2020 at 1:11 AM Martin KaFai Lau <kafai@xxxxxx> wrote:
>
> On Mon, Nov 16, 2020 at 11:25:35PM +0000, KP Singh wrote:
> > From: KP Singh <kpsingh@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > ---
> > 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.
Done.
>
> > */
> > #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.
Done.
>
> > +
> > +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.
Updated the docs:
* long bpf_lsm_set_bprm_opts(struct linux_binprm *bprm, u64 flags)
*
* Description
* Set or clear certain options on *bprm*:
*
* **BPF_LSM_F_BPRM_SECUREEXEC** Set the secureexec bit
* which sets the **AT_SECURE** auxv for glibc. The bit is
* is cleared if the flag is not specified.
* Return
* **-EINVAL** if invalid *flags* are passed.
>
> > + return 0;
> > +}
> > +
