netdev - [PATCH iproute2] ip: do not drop capabilities if net

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Message-Id: <20180511123956.5638-1-bluca@debian.org>
Date:   Fri, 11 May 2018 13:39:56 +0100
From:   Luca Boccassi <bluca@...ian.org>
To:     netdev@...r.kernel.org
Cc:     dsahern@...il.com, luto@...capital.net, stephen@...workplumber.org,
        Luca Boccassi <bluca@...ian.org>
Subject: [PATCH iproute2] ip: do not drop capabilities if net_admin=i is set

Users have reported a regression due to ip now dropping capabilities
unconditionally.
zerotier-one VPN and VirtualBox use ambient capabilities in their
binary and then fork out to ip to set routes and links, and this
does not work anymore.

As a workaround, do not drop caps if CAP_NET_ADMIN (the most common
capability used by ip) is set with the INHERITABLE flag.
Users that want ip vrf exec to work do not need to set INHERITABLE,
which will then only set when the calling program had privileges to
give itself the ambient capability.

Fixes: ba2fc55b99f8 ("Drop capabilities if not running ip exec vrf with libcap")

Signed-off-by: Luca Boccassi <bluca@...ian.org>
---

Reported on Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=898015
The reporter tested this patch and verified it solves the issue.

 lib/utils.c       | 15 ++++++++++++---
 man/man8/ip-vrf.8 |  4 ++++
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/lib/utils.c b/lib/utils.c
index 8a0bff0b..7b2c6dd1 100644
--- a/lib/utils.c
+++ b/lib/utils.c
@@ -1612,14 +1612,23 @@ void drop_cap(void)
 	/* 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_clear(capabilities) != 0)
-			exit(EXIT_FAILURE);
-		if (cap_set_proc(capabilities) != 0)
+		if (cap_get_flag(capabilities, net_admin, inheritable,
+		    &is_set) != 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
diff --git a/man/man8/ip-vrf.8 b/man/man8/ip-vrf.8
index 1a42cebe..c1c9b958 100644
--- a/man/man8/ip-vrf.8
+++ b/man/man8/ip-vrf.8
@@ -70,6 +70,10 @@ This command also requires to be ran as root or with the CAP_SYS_ADMIN,
 CAP_NET_ADMIN and CAP_DAC_OVERRIDE capabilities. If built with libcap and if
 capabilities are added to the ip binary program via setcap, the program will
 drop them as the first thing when invoked, unless the command is vrf exec.
+.br
+NOTE: capabilities will NOT be dropped if CAP_NET_ADMIN is set to INHERITABLE
+to avoid breaking programs with ambient capabilities that call ip.
+Do not set the INHERITABLE flag on the ip binary itself.
 
 .TP
 .B ip vrf identify [PID] - Report VRF association for process
-- 
2.14.2