[PATCH bpf-next 1/8] bpf: move unprivileged checks into map_create() and bpf_prog_load()

[Andrii Nakryiko]

On Wed, Apr 12, 2023 at 10:49 AM Kees Cook <keescook at chromium.org> wrote:
>
> On Tue, Apr 11, 2023 at 09:32:53PM -0700, Andrii Nakryiko wrote:
> > Make each bpf() syscall command a bit more self-contained, making it
> > easier to further enhance it. We move sysctl_unprivileged_bpf_disabled
> > handling down to map_create() and bpf_prog_load(), two special commands
> > in this regard.
> >
> > Signed-off-by: Andrii Nakryiko <andrii at kernel.org>
> > ---
> >  kernel/bpf/syscall.c | 37 ++++++++++++++++++++++---------------
> >  1 file changed, 22 insertions(+), 15 deletions(-)
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 6d575505f89c..c1d268025985 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -1130,6 +1130,17 @@ static int map_create(union bpf_attr *attr)
> >       int f_flags;
> >       int err;
> >
> > +     /* Intent here is for unprivileged_bpf_disabled to block key object
> > +      * creation commands for unprivileged users; other actions depend
> > +      * of fd availability and access to bpffs, so are dependent on
> > +      * object creation success.  Capabilities are later verified for
> > +      * operations such as load and map create, so even with unprivileged
> > +      * BPF disabled, capability checks are still carried out for these
> > +      * and other operations.
> > +      */
> > +     if (!bpf_capable() && sysctl_unprivileged_bpf_disabled)
> > +             return -EPERM;
>
> This appears to be a problem in the original code, but capability checks
> should be last, so that audit doesn't see a capability as having been
> used when it wasn't. i.e. if bpf_capable() passes, but
> sysctl_unprivileged_bpf_disabled isn't true, it'll look like a
> capability got used, and the flag gets set. Not a big deal at the end of
> the day, but the preferred ordering should be:
>
>         if (sysctl_unprivileged_bpf_disabled && !bpf_capable())
>                 ...
>

makes sense, I'll change the order



> > +
> >       err = CHECK_ATTR(BPF_MAP_CREATE);
> >       if (err)
> >               return -EINVAL;
> > @@ -2512,6 +2523,17 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
> >       char license[128];
> >       bool is_gpl;
> >
> > +     /* Intent here is for unprivileged_bpf_disabled to block key object
> > +      * creation commands for unprivileged users; other actions depend
> > +      * of fd availability and access to bpffs, so are dependent on
> > +      * object creation success.  Capabilities are later verified for
> > +      * operations such as load and map create, so even with unprivileged
> > +      * BPF disabled, capability checks are still carried out for these
> > +      * and other operations.
> > +      */
> > +     if (!bpf_capable() && sysctl_unprivileged_bpf_disabled)
> > +             return -EPERM;
> > +
> >       if (CHECK_ATTR(BPF_PROG_LOAD))
> >               return -EINVAL;
> >
> > @@ -5008,23 +5030,8 @@ static int bpf_prog_bind_map(union bpf_attr *attr)
> >  static int __sys_bpf(int cmd, bpfptr_t uattr, unsigned int size)
> >  {
> >       union bpf_attr attr;
> > -     bool capable;
> >       int err;
> >
> > -     capable = bpf_capable() || !sysctl_unprivileged_bpf_disabled;
> > -
> > -     /* Intent here is for unprivileged_bpf_disabled to block key object
> > -      * creation commands for unprivileged users; other actions depend
> > -      * of fd availability and access to bpffs, so are dependent on
> > -      * object creation success.  Capabilities are later verified for
> > -      * operations such as load and map create, so even with unprivileged
> > -      * BPF disabled, capability checks are still carried out for these
> > -      * and other operations.
> > -      */
> > -     if (!capable &&
> > -         (cmd == BPF_MAP_CREATE || cmd == BPF_PROG_LOAD))
> > -             return -EPERM;
> > -
> >       err = bpf_check_uarg_tail_zero(uattr, sizeof(attr), size);
> >       if (err)
> >               return err;
> > --
> > 2.34.1
> >
>
> --
> Kees Cook