[PATCH v5 bpf-next 01/13] bpf: align CAP_NET_ADMIN checks with bpf_capable() approach

[Andrii Nakryiko]

Within BPF syscall handling code CAP_NET_ADMIN checks stand out a bit
compared to CAP_BPF and CAP_PERFMON checks. For the latter, CAP_BPF or
CAP_PERFMON are checked first, but if they are not set, CAP_SYS_ADMIN
takes over and grants whatever part of BPF syscall is required.

Similar kind of checks that involve CAP_NET_ADMIN are not so consistent.
One out of four uses does follow CAP_BPF/CAP_PERFMON model: during
BPF_PROG_LOAD, if the type of BPF program is "network-related" either
CAP_NET_ADMIN or CAP_SYS_ADMIN is required to proceed.

But in three other cases CAP_NET_ADMIN is required even if CAP_SYS_ADMIN
is set:
  - when creating DEVMAP/XDKMAP/CPU_MAP maps;
  - when attaching CGROUP_SKB programs;
  - when handling BPF_PROG_QUERY command.

This patch is changing the latter three cases to follow BPF_PROG_LOAD
model, that is allowing to proceed under either CAP_NET_ADMIN or
CAP_SYS_ADMIN.

This also makes it cleaner in subsequent BPF token patches to switch
wholesomely to a generic bpf_token_capable(int cap) check, that always
falls back to CAP_SYS_ADMIN if requested capability is missing.

Cc: Jakub Kicinski <kuba at kernel.org>
Signed-off-by: Andrii Nakryiko <andrii at kernel.org>
---
 kernel/bpf/syscall.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 85c1d908f70f..f024caee0bba 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1097,6 +1097,11 @@ static int map_check_btf(struct bpf_map *map, const struct btf *btf,
 	return ret;
 }
 
+static bool bpf_net_capable(void)
+{
+	return capable(CAP_NET_ADMIN) || capable(CAP_SYS_ADMIN);
+}
+
 #define BPF_MAP_CREATE_LAST_FIELD map_extra
 /* called via syscall */
 static int map_create(union bpf_attr *attr)
@@ -1200,7 +1205,7 @@ static int map_create(union bpf_attr *attr)
 	case BPF_MAP_TYPE_DEVMAP:
 	case BPF_MAP_TYPE_DEVMAP_HASH:
 	case BPF_MAP_TYPE_XSKMAP:
-		if (!capable(CAP_NET_ADMIN))
+		if (!bpf_net_capable())
 			return -EPERM;
 		break;
 	default:
@@ -2595,7 +2600,7 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
 	    !bpf_capable())
 		return -EPERM;
 
-	if (is_net_admin_prog_type(type) && !capable(CAP_NET_ADMIN) && !capable(CAP_SYS_ADMIN))
+	if (is_net_admin_prog_type(type) && !bpf_net_capable())
 		return -EPERM;
 	if (is_perfmon_prog_type(type) && !perfmon_capable())
 		return -EPERM;
@@ -3738,7 +3743,7 @@ static int bpf_prog_attach_check_attach_type(const struct bpf_prog *prog,
 	case BPF_PROG_TYPE_SK_LOOKUP:
 		return attach_type == prog->expected_attach_type ? 0 : -EINVAL;
 	case BPF_PROG_TYPE_CGROUP_SKB:
-		if (!capable(CAP_NET_ADMIN))
+		if (!bpf_net_capable())
 			/* cg-skb progs can be loaded by unpriv user.
 			 * check permissions at attach time.
 			 */
@@ -3922,7 +3927,7 @@ static int bpf_prog_detach(const union bpf_attr *attr)
 static int bpf_prog_query(const union bpf_attr *attr,
 			  union bpf_attr __user *uattr)
 {
-	if (!capable(CAP_NET_ADMIN))
+	if (!bpf_net_capable())
 		return -EPERM;
 	if (CHECK_ATTR(BPF_PROG_QUERY))
 		return -EINVAL;
-- 
2.34.1