netdev - Simplify ambient capability dropping in iproute2:ip tool.

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Date:   Thu, 28 Apr 2022 20:17:51 -0700
From:   Tinkerer One <tinkerer@...pem.net>
To:     netdev@...r.kernel.org
Cc:     bluca@...ian.org
Subject: Simplify ambient capability dropping in iproute2:ip tool.

Hi,

This is expanded from https://github.com/shemminger/iproute2/issues/62
which I'm told is not the way to report issues and offer fixes to
iproute2 etc.

[I'm not subscribed to the netdev list, so please cc: me if you need more info.]

The original change that added the drop_cap() code was:

https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/commit/?id=ba2fc55b99f8363c80ce36681bc1ec97690b66f5

In an attempt to address some user feedback, the code was further
complicated by:

https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/commit/?id=9b13cc98f5952f62b825461727c8170d37a4037d

Another user issue was asked about here (a couple days ago):

https://stackoverflow.com/questions/72015197/allow-non-root-user-of-container-to-execute-binaries-that-need-capabilities

I looked into what was going on and found that lib/utils.c contains
some complicated code that seems to be trying to prevent Ambient
capabilities from being inherited except in specific cases
(ip/ip.c:main() calls drop_cap() except in the ip vrf exec case.). The
code clears all capabilities in order to prevent Ambient capabilities
from being available. The following change achieves suppression of
Ambient capabilities much more precisely. It also permits ip to not
need to be setuid-root or executed under sudo since it can now be
optionally empowered by file capabilities:

diff --git a/lib/utils.c b/lib/utils.c
index 53d31006..681e4aee 100644
--- a/lib/utils.c
+++ b/lib/utils.c
@@ -1555,25 +1555,10 @@ void drop_cap(void)
 #ifdef HAVE_LIBCAP
        /* don't harmstring root/sudo */
        if (getuid() != 0 && geteuid() != 0) {
-               cap_t capabilities;
-               cap_value_t net_admin = CAP_NET_ADMIN;
-               cap_flag_t inheritable = CAP_INHERITABLE;
-               cap_flag_value_t is_set;
-
-               capabilities = cap_get_proc();
-               if (!capabilities)
-                       exit(EXIT_FAILURE);
-               if (cap_get_flag(capabilities, net_admin, inheritable,
-                   &is_set) != 0)
+               /* prevent any ambient capabilities from being inheritable */
+               if (cap_reset_ambient() != 0) {
                        exit(EXIT_FAILURE);
-               /* apps with ambient caps can fork and call ip */
-               if (is_set == CAP_CLEAR) {
-                       if (cap_clear(capabilities) != 0)
-                               exit(EXIT_FAILURE);
-                       if (cap_set_proc(capabilities) != 0)
-                               exit(EXIT_FAILURE);
                }
-               cap_free(capabilities);
        }
 #endif
 }