Re: [PATCH v4 bpf-next 1/4] capability: introduce CAP_BPF and CAP_TRACING
From: Andy Lutomirski <luto@kernel.org>
Date: 2019-09-09 22:52:42
Also in:
bpf, linux-api, linux-security-module
On Fri, Sep 6, 2019 at 4:10 PM Alexei Starovoitov [off-list ref] wrote:
Split BPF and perf/tracing operations that are allowed under
CAP_SYS_ADMIN into corresponding CAP_BPF and CAP_TRACING.
For backward compatibility include them in CAP_SYS_ADMIN as well.
The end result provides simple safety model for applications that use BPF:
- for tracing program types
BPF_PROG_TYPE_{KPROBE, TRACEPOINT, PERF_EVENT, RAW_TRACEPOINT, etc}
use CAP_BPF and CAP_TRACING
- for networking program types
BPF_PROG_TYPE_{SCHED_CLS, XDP, CGROUP_SKB, SK_SKB, etc}
use CAP_BPF and CAP_NET_ADMIN
There are few exceptions from this simple rule:
- bpf_trace_printk() is allowed in networking programs, but it's using
ftrace mechanism, hence this helper needs additional CAP_TRACING.
- cpumap is used by XDP programs. Currently it's kept under CAP_SYS_ADMIN,
but could be relaxed to CAP_NET_ADMIN in the future.
- BPF_F_ZERO_SEED flag for hash/lru map is allowed under CAP_SYS_ADMIN only
to discourage production use.
- BPF HW offload is allowed under CAP_SYS_ADMIN.
- cg_sysctl, cg_device, lirc program types are neither networking nor tracing.
They can be loaded under CAP_BPF, but attach is allowed under CAP_NET_ADMIN.
This will be cleaned up in the future.
userid=nobody + (CAP_TRACING | CAP_NET_ADMIN) + CAP_BPF is safer than
typical setup with userid=root and sudo by existing bpf applications.
It's not secure, since these capabilities:
- allow bpf progs access arbitrary memory
- let tasks access any bpf map
- let tasks attach/detach any bpf prog
bpftool, bpftrace, bcc tools binaries should not be installed with
cap_bpf+cap_tracing, since unpriv users will be able to read kernel secrets.
CAP_BPF, CAP_NET_ADMIN, CAP_TRACING are roughly equal in terms of
damage they can make to the system.
Example:
CAP_NET_ADMIN can stop network traffic. CAP_BPF can write into map
and if that map is used by firewall-like bpf prog the network traffic
may stop.
CAP_BPF allows many bpf prog_load commands in parallel. The verifier
may consume large amount of memory and significantly slow down the system.
CAP_TRACING allows many kprobes that can slow down the system.Do we want to split CAP_TRACE_KERNEL and CAP_TRACE_USER? It's not entirely clear to me that it's useful.
quoted hunk ↗ jump to hunk
In the future more fine-grained bpf permissions may be added. Existing unprivileged BPF operations are not affected. In particular unprivileged users are allowed to load socket_filter and cg_skb program types and to create array, hash, prog_array, map-in-map map types. Signed-off-by: Alexei Starovoitov <ast@kernel.org> --- include/linux/capability.h | 18 +++++++++++ include/uapi/linux/capability.h | 49 ++++++++++++++++++++++++++++- security/selinux/include/classmap.h | 4 +-- 3 files changed, 68 insertions(+), 3 deletions(-)diff --git a/include/linux/capability.h b/include/linux/capability.h index ecce0f43c73a..13eb49c75797 100644 --- a/include/linux/capability.h +++ b/include/linux/capability.h@@ -247,6 +247,24 @@ static inline bool ns_capable_setid(struct user_namespace *ns, int cap) return true; } #endif /* CONFIG_MULTIUSER */ + +static inline bool capable_bpf(void) +{ + return capable(CAP_SYS_ADMIN) || capable(CAP_BPF); +} +static inline bool capable_tracing(void) +{ + return capable(CAP_SYS_ADMIN) || capable(CAP_TRACING); +} +static inline bool capable_bpf_tracing(void) +{ + return capable(CAP_SYS_ADMIN) || (capable(CAP_BPF) && capable(CAP_TRACING)); +} +static inline bool capable_bpf_net_admin(void) +{ + return (capable(CAP_SYS_ADMIN) || capable(CAP_BPF)) && capable(CAP_NET_ADMIN); +} +
These helpers are all wrong, unfortunately, since they will produce inappropriate audit events. capable_bpf() should look more like this: if (capable_noaudit(CAP_BPF)) return capable(CAP_BPF); if (capable_noaudit(CAP_SYS_ADMIN)) return capable(CAP_SYS_ADMIN); return capable(CAP_BPF); James, etc: should there instead be new helpers to do this more generically rather than going through the noaudit contortions? My code above is horrible.