On Wed, Apr 12, 2023 at 10:49 AM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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
