[Openvpn-devel,v3] platform: Retain CAP_NET_ADMIN when dropping privileges

Message ID 20220407184023.249-1-timo@rothenpieler.org
State Superseded
Headers show
Series
  • [Openvpn-devel,v3] platform: Retain CAP_NET_ADMIN when dropping privileges
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Commit Message

Timo Rothenpieler April 7, 2022, 6:40 p.m.
On Linux, when dropping privileges, interaction with
the network configuration, such as tearing down routes
or ovpn-dco interfaces will fail when --user/--group are
used.

This patch sets the CAP_NET_ADMIN capability, which grants
the needed privileges during the lifetime of the OpenVPN
process when dropping root privileges.

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>
Reviewed-By: David Sommerseth <davids@openvpn.net>
---
 configure.ac                              |  19 ++++
 distro/systemd/openvpn-client@.service.in |   2 +-
 distro/systemd/openvpn-server@.service.in |   2 +-
 src/openvpn/init.c                        |  30 ++++++-
 src/openvpn/platform.c                    | 105 +++++++++++++++++++++-
 src/openvpn/platform.h                    |   7 +-
 6 files changed, 156 insertions(+), 9 deletions(-)

Comments

Gert Doering April 8, 2022, 9:35 a.m. | #1
Hi,

On Thu, Apr 07, 2022 at 08:40:24PM +0200, Timo Rothenpieler wrote:
> +    else if (res < 0)
> +    {
> +        if (res == -3)
> +        {
> +            msg(M_NONFATAL, "Following error likely due to missing capability CAP_SETPCAP.");
> +        }
> +        msg(err_flags | M_ERRNO, "capng_change_id('%s','%s') failed retaining capabilities: %d",
> +            user_state->username, group_state->groupname, res);
> +        goto fallback;
> +    }

Wouldn't that overwrite errno for the "res == -3" case, given that
msg() will do stdio stuff?  Maybe reorder and print the "error likely due..."
message with a preceding "NOTE:" after the capng_change_id() message?

(That would be more typical for our logs - the "NOTE: this could be
because..." tends to come after the error message)

> +    if (new_uid >= 0)
> +    {
> +         msg(M_INFO, "UID set to %s", user_state->username);
> +    }
> +    if (new_gid >= 0)
> +    {
> +         msg(M_INFO, "GID set to %s", group_state->groupname);
> +    }
> +
> +    msg(M_INFO, "Capabilities retained: CAP_NET_ADMIN");
> +
> +    return;
> +fallback:

My inner whitespace dragon does would prefer to have the blank line
between "return" and "fallback:" (and no blank linke after the M_INFO).

> +    /* capng_change_id() can leave this flag clobbered on failure
> +     * This is working around a bug in libcap-ng, which can leave the flag set
> +     * on failure: https://github.com/stevegrubb/libcap-ng/issues/33 */
> +    if (prctl(PR_GET_KEEPCAPS) && prctl(PR_SET_KEEPCAPS, 0) < 0)
> +    {
> +        msg(M_ERR, "Clearing KEEPCAPS flag failed");
> +    }
> +#endif  /* HAVE_LIBCAPNG */

This one does not really look like it should be in "fallback:" - because
that way it always gets called, even if we jump there right at function
entry, if keep_caps == 0.

> +
> +    if (keep_caps)
> +    {
> +        msg(err_flags, "Unable to retain capabilities");
> +    }
> +
> +    platform_group_set(group_state);
> +    platform_user_set(user_state);
> +}
> +

Maybe "fallback:" should be right before platform_group_set()?


(Sorry for being late to the "complain about your code" party...)

gert
Timo Rothenpieler April 8, 2022, 11:18 a.m. | #2
On 08/04/2022 11:35, Gert Doering wrote:
> Hi,
> 
> On Thu, Apr 07, 2022 at 08:40:24PM +0200, Timo Rothenpieler wrote:
>> +    else if (res < 0)
>> +    {
>> +        if (res == -3)
>> +        {
>> +            msg(M_NONFATAL, "Following error likely due to missing capability CAP_SETPCAP.");
>> +        }
>> +        msg(err_flags | M_ERRNO, "capng_change_id('%s','%s') failed retaining capabilities: %d",
>> +            user_state->username, group_state->groupname, res);
>> +        goto fallback;
>> +    }
> 
> Wouldn't that overwrite errno for the "res == -3" case, given that
> msg() will do stdio stuff?  Maybe reorder and print the "error likely due..."
> message with a preceding "NOTE:" after the capng_change_id() message?
> 
> (That would be more typical for our logs - the "NOTE: this could be
> because..." tends to come after the error message)

Good point.
I like that idea better as well, will implement.

>> +    if (new_uid >= 0)
>> +    {
>> +         msg(M_INFO, "UID set to %s", user_state->username);
>> +    }
>> +    if (new_gid >= 0)
>> +    {
>> +         msg(M_INFO, "GID set to %s", group_state->groupname);
>> +    }
>> +
>> +    msg(M_INFO, "Capabilities retained: CAP_NET_ADMIN");
>> +
>> +    return;
>> +fallback:
> 
> My inner whitespace dragon does would prefer to have the blank line
> between "return" and "fallback:" (and no blank linke after the M_INFO).

No strong preference there on my side, so sure.

>> +    /* capng_change_id() can leave this flag clobbered on failure
>> +     * This is working around a bug in libcap-ng, which can leave the flag set
>> +     * on failure: https://github.com/stevegrubb/libcap-ng/issues/33 */
>> +    if (prctl(PR_GET_KEEPCAPS) && prctl(PR_SET_KEEPCAPS, 0) < 0)
>> +    {
>> +        msg(M_ERR, "Clearing KEEPCAPS flag failed");
>> +    }
>> +#endif  /* HAVE_LIBCAPNG */
> 
> This one does not really look like it should be in "fallback:" - because
> that way it always gets called, even if we jump there right at function
> entry, if keep_caps == 0.

No, it's intentional. It ensures that it's printed even if we don't 
HAVE_LIBCAPNG but someone called the function requesting to keep 
capabilities, which is not possible then.

>> +
>> +    if (keep_caps)
>> +    {
>> +        msg(err_flags, "Unable to retain capabilities");
>> +    }
>> +
>> +    platform_group_set(group_state);
>> +    platform_user_set(user_state);
>> +}
>> +
> 
> Maybe "fallback:" should be right before platform_group_set()?
> 
> 
> (Sorry for being late to the "complain about your code" party...)
> 
> gert
Antonio Quartulli April 8, 2022, 11:25 a.m. | #3
Hi,

On 08/04/2022 13:18, Timo Rothenpieler wrote:
>> This one does not really look like it should be in "fallback:" - because
>> that way it always gets called, even if we jump there right at function
>> entry, if keep_caps == 0.
> 
> No, it's intentional. It ensures that it's printed even if we don't 
> HAVE_LIBCAPNG but someone called the function requesting to keep 
> capabilities, which is not possible then.

Along Gert's thoughts: maybe there should be 2 labels, one that is just 
"out" and one that is "fallback".

Simply because jumping to "fallback" when it was not requested to retain 
capabilities (keep_caps == 0) may not sound right (are we really falling 
back in this case?)

Cheers,

> 
>>> +
>>> +    if (keep_caps)
>>> +    {
>>> +        msg(err_flags, "Unable to retain capabilities");
>>> +    }
>>> +
>>> +    platform_group_set(group_state);
>>> +    platform_user_set(user_state);
>>> +}
>>> +
>>
>> Maybe "fallback:" should be right before platform_group_set()?
>>
>>
>> (Sorry for being late to the "complain about your code" party...)
>>
>> gert
> 
> 
> _______________________________________________
> Openvpn-devel mailing list
> Openvpn-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/openvpn-devel
>

Patch

diff --git a/configure.ac b/configure.ac
index 85921ddb..d2eb3426 100644
--- a/configure.ac
+++ b/configure.ac
@@ -794,6 +794,25 @@  dnl
 	esac
 fi
 
+dnl
+dnl Depend on libcap-ng on Linux
+dnl
+case "$host" in
+	*-*-linux*)
+		PKG_CHECK_MODULES([LIBCAPNG],
+				  [libcap-ng],
+				  [],
+				  [AC_MSG_ERROR([libcap-ng package not found. Is the development package and pkg-config installed?])]
+		)
+		AC_CHECK_HEADER([sys/prctl.h],,[AC_MSG_ERROR([sys/prctl.h not found!])])
+
+		CFLAGS="${CFLAGS} ${LIBCAPNG_CFALGS}"
+		LIBS="${LIBS} ${LIBCAPNG_LIBS}"
+		AC_DEFINE(HAVE_LIBCAPNG, 1, [Enable libcap-ng support])
+	;;
+esac
+
+
 if test "${with_crypto_library}" = "openssl"; then
 	AC_ARG_VAR([OPENSSL_CFLAGS], [C compiler flags for OpenSSL])
 	AC_ARG_VAR([OPENSSL_LIBS], [linker flags for OpenSSL])
diff --git a/distro/systemd/openvpn-client@.service.in b/distro/systemd/openvpn-client@.service.in
index cbcef653..159fb4dc 100644
--- a/distro/systemd/openvpn-client@.service.in
+++ b/distro/systemd/openvpn-client@.service.in
@@ -11,7 +11,7 @@  Type=notify
 PrivateTmp=true
 WorkingDirectory=/etc/openvpn/client
 ExecStart=@sbindir@/openvpn --suppress-timestamps --nobind --config %i.conf
-CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SYS_CHROOT CAP_DAC_OVERRIDE
+CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SETPCAP CAP_SYS_CHROOT CAP_DAC_OVERRIDE
 LimitNPROC=10
 DeviceAllow=/dev/null rw
 DeviceAllow=/dev/net/tun rw
diff --git a/distro/systemd/openvpn-server@.service.in b/distro/systemd/openvpn-server@.service.in
index d1cc72cb..6e8e7d94 100644
--- a/distro/systemd/openvpn-server@.service.in
+++ b/distro/systemd/openvpn-server@.service.in
@@ -11,7 +11,7 @@  Type=notify
 PrivateTmp=true
 WorkingDirectory=/etc/openvpn/server
 ExecStart=@sbindir@/openvpn --status %t/openvpn-server/status-%i.log --status-version 2 --suppress-timestamps --config %i.conf
-CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SYS_CHROOT CAP_DAC_OVERRIDE CAP_AUDIT_WRITE
+CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SETPCAP CAP_SYS_CHROOT CAP_DAC_OVERRIDE CAP_AUDIT_WRITE
 LimitNPROC=10
 DeviceAllow=/dev/null rw
 DeviceAllow=/dev/net/tun rw
diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index 8818ba6f..0a1b00ca 100644
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -1138,6 +1138,30 @@  possibly_become_daemon(const struct options *options)
     return ret;
 }
 
+/*
+ * Determine if we need to retain process capabilities. DCO and SITNL need it.
+ * Enforce it for DCO, but only try and soft-fail for SITNL to keep backwards compat.
+ *
+ * Returns the tri-state expected by platform_user_group_set.
+ * -1: try to keep caps, but continue if impossible
+ *  0: don't keep caps
+ *  1: keep caps, fail hard if impossible
+ */
+static int
+need_keep_caps(struct context *c)
+{
+    if (dco_enabled(&c->options))
+    {
+        return 1;
+    }
+
+#ifdef ENABLE_SITNL
+    return -1;
+#else
+    return 0;
+#endif
+}
+
 /*
  * Actually do UID/GID downgrade, chroot and SELinux context switching, if requested.
  */
@@ -1167,8 +1191,10 @@  do_uid_gid_chroot(struct context *c, bool no_delay)
         {
             if (no_delay)
             {
-                platform_group_set(&c0->platform_state_group);
-                platform_user_set(&c0->platform_state_user);
+                int keep_caps = need_keep_caps(c);
+                platform_user_group_set(&c0->platform_state_user,
+                                        &c0->platform_state_group,
+                                        keep_caps);
             }
             else if (c->first_time)
             {
diff --git a/src/openvpn/platform.c b/src/openvpn/platform.c
index 450f28ba..1e321369 100644
--- a/src/openvpn/platform.c
+++ b/src/openvpn/platform.c
@@ -43,6 +43,11 @@ 
 #include <direct.h>
 #endif
 
+#ifdef HAVE_LIBCAPNG
+#include <cap-ng.h>
+#include <sys/prctl.h>
+#endif
+
 /* Redefine the top level directory of the filesystem
  * to restrict access to files for security */
 void
@@ -91,7 +96,7 @@  platform_user_get(const char *username, struct platform_state_user *state)
     return ret;
 }
 
-void
+static void
 platform_user_set(const struct platform_state_user *state)
 {
 #if defined(HAVE_GETPWNAM) && defined(HAVE_SETUID)
@@ -130,7 +135,7 @@  platform_group_get(const char *groupname, struct platform_state_group *state)
     return ret;
 }
 
-void
+static void
 platform_group_set(const struct platform_state_group *state)
 {
 #if defined(HAVE_GETGRNAM) && defined(HAVE_SETGID)
@@ -155,6 +160,102 @@  platform_group_set(const struct platform_state_group *state)
 #endif
 }
 
+/* Set user and group, retaining neccesary capabilities required by the platform.
+ *
+ * The keep_caps argument has 3 possible states:
+ *  >0: Retain capabilities, and fail hard on failure to do so.
+ * ==0: Don't attempt to retain any capabilities, just sitch user/group.
+ *  <0: Try to retain capabilities, but continue on failure.
+ */
+void platform_user_group_set(const struct platform_state_user *user_state,
+                             const struct platform_state_group *group_state,
+                             int keep_caps)
+{
+    unsigned int err_flags = (keep_caps > 0) ? M_FATAL : M_NONFATAL;
+#ifdef HAVE_LIBCAPNG
+    int new_gid = -1, new_uid = -1;
+    int res;
+
+    if (keep_caps == 0)
+    {
+        goto fallback;
+    }
+
+    /*
+     * new_uid/new_gid defaults to -1, which will not make
+     * libcap-ng change the UID/GID unless configured
+     */
+    if (group_state->groupname && group_state->gr)
+    {
+        new_gid = group_state->gr->gr_gid;
+    }
+    if (user_state->username && user_state->pw)
+    {
+        new_uid = user_state->pw->pw_uid;
+    }
+
+    /* Prepare capabilities before dropping UID/GID */
+    capng_clear(CAPNG_SELECT_BOTH);
+    res = capng_update(CAPNG_ADD, CAPNG_EFFECTIVE | CAPNG_PERMITTED, CAP_NET_ADMIN);
+    if (res < 0)
+    {
+        msg(err_flags, "capng_update(CAP_NET_ADMIN) failed: %d", res);
+        goto fallback;
+    }
+
+    /* Change to new UID/GID.
+     * capng_change_id() internally calls capng_apply() to apply prepared capabilities.
+     */
+    res = capng_change_id(new_uid, new_gid, CAPNG_DROP_SUPP_GRP | CAPNG_CLEAR_BOUNDING);
+    if (res == -4 || res == -6)
+    {
+        /* -4 and -6 mean failure of setuid/gid respectively.
+           There is no point for us to continue if those failed. */
+        msg(M_ERR, "capng_change_id('%s','%s') failed: %d",
+            user_state->username, group_state->groupname, res);
+    }
+    else if (res < 0)
+    {
+        if (res == -3)
+        {
+            msg(M_NONFATAL, "Following error likely due to missing capability CAP_SETPCAP.");
+        }
+        msg(err_flags | M_ERRNO, "capng_change_id('%s','%s') failed retaining capabilities: %d",
+            user_state->username, group_state->groupname, res);
+        goto fallback;
+    }
+
+    if (new_uid >= 0)
+    {
+         msg(M_INFO, "UID set to %s", user_state->username);
+    }
+    if (new_gid >= 0)
+    {
+         msg(M_INFO, "GID set to %s", group_state->groupname);
+    }
+
+    msg(M_INFO, "Capabilities retained: CAP_NET_ADMIN");
+
+    return;
+fallback:
+    /* capng_change_id() can leave this flag clobbered on failure
+     * This is working around a bug in libcap-ng, which can leave the flag set
+     * on failure: https://github.com/stevegrubb/libcap-ng/issues/33 */
+    if (prctl(PR_GET_KEEPCAPS) && prctl(PR_SET_KEEPCAPS, 0) < 0)
+    {
+        msg(M_ERR, "Clearing KEEPCAPS flag failed");
+    }
+#endif  /* HAVE_LIBCAPNG */
+
+    if (keep_caps)
+    {
+        msg(err_flags, "Unable to retain capabilities");
+    }
+
+    platform_group_set(group_state);
+    platform_user_set(user_state);
+}
+
 /* Change process priority */
 void
 platform_nice(int niceval)
diff --git a/src/openvpn/platform.h b/src/openvpn/platform.h
index a3eec298..19187d3a 100644
--- a/src/openvpn/platform.h
+++ b/src/openvpn/platform.h
@@ -79,11 +79,12 @@  struct platform_state_group {
 
 bool platform_user_get(const char *username, struct platform_state_user *state);
 
-void platform_user_set(const struct platform_state_user *state);
-
 bool platform_group_get(const char *groupname, struct platform_state_group *state);
 
-void platform_group_set(const struct platform_state_group *state);
+void platform_user_group_set(const struct platform_state_user *user_state,
+                             const struct platform_state_group *group_state,
+                             int keep_caps);
+
 
 /*
  * Extract UID or GID