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())
...
> +
> 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
