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

[Kees Cook <keescook@chromium.org>]

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@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())
		...

> +
>  	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