Re: [PATCH bpf-next v2 1/2] bpf: Add bpf_lsm_set_bprm_opts helper

[KP Singh]

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;
> > +}
> > +