Re: [PATCH bpf-next v3 1/2] bpf: Add bpf_lsm_set_bprm_opts helper

[KP Singh]

On Tue, Nov 17, 2020 at 11:41 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 11/17/20 3:13 AM, 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       | 18 ++++++++++++++++++
> >   kernel/bpf/bpf_lsm.c           | 27 +++++++++++++++++++++++++++
> >   scripts/bpf_helpers_doc.py     |  2 ++
> >   tools/include/uapi/linux/bpf.h | 18 ++++++++++++++++++
> >   4 files changed, 65 insertions(+)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 162999b12790..bfa79054d106 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3787,6 +3787,18 @@ 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)
> > + *
>
> small nit: should have no extra newline (same for the tools/ copy)
>
> > + *   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 cleared if the flag is not specified.
> > + *   Return
> > + *           **-EINVAL** if invalid *flags* are passed.
> > + *
> >    */
> >   #define __BPF_FUNC_MAPPER(FN)               \
> >       FN(unspec),                     \
> > @@ -3948,6 +3960,7 @@ 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 +4132,11 @@ 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..cd85482228a0 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     BPF_LSM_F_BPRM_SECUREEXEC
> > +
> > +BPF_CALL_2(bpf_lsm_set_bprm_opts, struct linux_binprm *, bprm, u64, flags)
> > +{
> > +
>
> ditto
>
> Would have fixed up these things on the fly while applying, but one small item
> I wanted to bring up here given uapi which will then freeze: it would be cleaner
> to call the helper just bpf_bprm_opts_set() or so given it's implied that we
> attach to lsm here and we don't use _lsm in the naming for the others either.
> Similarly, I'd drop the _LSM from the flag/mask.
>

Thanks Daniel, this makes sense and is more future proof, I respun this and
sent out another version with some minor fixes and the rename. Also added
Martin's acks.

- KP