[PATCH v3 6/6] setpriv: allow modifying ambient capabilities

[Patrick Steinhardt]

With Linux 4.3, a new set of capabilities has been introduced with the
ambient capabilities. These aim to solve the problem that it was
impossible to grant run programs with elevated privileges across
non-root users. Quoting from capabilities(7):

    This is a set of capabilities that are preserved across an execve(2)
    of a program that is not privileged.  The ambient capability set
    obeys the invariant that no capability can ever be ambient if it is
    not both permitted and inheritable.

With this new set of capabilities it is now possible to run an
executable with elevated privileges as a different user, making it much
easier to do proper privilege separation.

Note though that the `--ambient-caps` switch is not part of any released
version of util-linux, yet. It has been applied in 0c92194ee (setpriv:
support modifying the set of ambient capabilities, 2017-06-24) and will
probably be part of v2.31.

function                                             old     new   delta
parse_cap                                              -     174    +174
setpriv_main                                        1246    1301     +55
.rodata                                           146307  146347     +40
static.setpriv_longopts                               40      55     +15
packed_usage                                       32092   32079     -13

Signed-off-by: Patrick Steinhardt <ps at pks.im>
---
 util-linux/setpriv.c | 49 +++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 43 insertions(+), 6 deletions(-)

diff --git a/util-linux/setpriv.c b/util-linux/setpriv.c
index b9e481a6f..9ada33df4 100644
--- a/util-linux/setpriv.c
+++ b/util-linux/setpriv.c
@@ -53,16 +53,20 @@
 //usage:#define setpriv_full_usage "\n\n"
 //usage:       "Run PROG with different privilege settings\n"
 //usage:	IF_FEATURE_SETPRIV_DUMP(
-//usage:     "\n-d,--dump		Show current capabilities"
+//usage:     "\n-d,--dump			Show current capabilities"
 //usage:	)
-//usage:     "\n--nnp,--no-new-privs	Ignore setuid/setgid bits and file capabilities"
+//usage:     "\n--nnp,--no-new-privs		Ignore setuid/setgid bits and file capabilities"
 //usage:	IF_FEATURE_SETPRIV_CAPABILITIES(
-//usage:     "\n--inh-caps <caps,...>	Set inheritable capabilities"
+//usage:     "\n--inh-caps <caps,...>		Set inheritable capabilities"
+//usage:	)
+//usage:	IF_FEATURE_SETPRIV_CAPABILITIES(
+//usage:     "\n--ambient-caps <caps,...>	Set ambient capabilities"
 //usage:	)
 
-//setpriv from util-linux 2.28:
+//setpriv from util-linux 4fb515f90079739771b64dcb6beeb384e3be7cf5
 // -d, --dump               show current state (and do not exec anything)
 // --nnp, --no-new-privs    disallow granting new privileges
+// --ambient-caps <caps,...> set ambient capabilities
 // --inh-caps <caps,...>    set inheritable capabilities
 // --bounding-set <caps>    set capability bounding set
 // --ruid <uid>             set real uid
@@ -100,15 +104,19 @@
 #ifndef PR_CAP_AMBIENT
 #define PR_CAP_AMBIENT 47
 #define PR_CAP_AMBIENT_IS_SET 1
+#define PR_CAP_AMBIENT_RAISE 2
+#define PR_CAP_AMBIENT_LOWER 3
 #endif
 
 enum {
 	IF_FEATURE_SETPRIV_DUMP(OPTBIT_DUMP,)
 	IF_FEATURE_SETPRIV_CAPABILITIES(OPTBIT_INH,)
+	IF_FEATURE_SETPRIV_CAPABILITIES(OPTBIT_AMB,)
 	OPTBIT_NNP,
 
 	IF_FEATURE_SETPRIV_DUMP(OPT_DUMP = (1 << OPTBIT_DUMP),)
 	IF_FEATURE_SETPRIV_CAPABILITIES(OPT_INH  = (1 << OPTBIT_INH),)
+	IF_FEATURE_SETPRIV_CAPABILITIES(OPT_AMB  = (1 << OPTBIT_AMB),)
 	OPT_NNP  = (1 << OPTBIT_NNP),
 };
 
@@ -396,6 +404,29 @@ static void set_inh_caps(char *capstring)
 	if (ENABLE_FEATURE_CLEAN_UP)
 		free(caps.data);
 }
+
+static void set_ambient_caps(char *string)
+{
+	char *cap;
+
+	cap = strtok(string, ",");
+	while (cap) {
+		unsigned long index;
+		int add;
+
+		parse_cap(&index, &add, cap);
+
+		if (add) {
+			if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, index, 0, 0) < 0)
+				bb_perror_msg("cap_ambient_raise");
+		} else {
+			if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_LOWER, index, 0, 0) < 0)
+				bb_perror_msg("cap_ambient_lower");
+		}
+
+		cap = strtok(NULL, ",");
+	}
+}
 #endif /* FEATURE_SETPRIV_CAPABILITIES */
 
 int setpriv_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
@@ -410,15 +441,18 @@ int setpriv_main(int argc UNUSED_PARAM, char **argv)
 		IF_FEATURE_SETPRIV_CAPABILITIES(
 		"inh-caps\0"     Required_argument	"\xfe"
 		)
+		IF_FEATURE_SETPRIV_CAPABILITIES(
+		"ambient-caps\0" Required_argument	"\xfd"
+		)
 		;
 #if ENABLE_FEATURE_SETPRIV_CAPABILITIES
-	char *inh_caps;
+	char *inh_caps, *ambient_caps;
 #endif
 	int opts;
 
 	applet_long_options = setpriv_longopts;
 	opts = getopt32(argv, "+"IF_FEATURE_SETPRIV_DUMP("d")
-			IF_FEATURE_SETPRIV_CAPABILITIES("\xfe:" , &inh_caps));
+			IF_FEATURE_SETPRIV_CAPABILITIES("\xfe:\xfd:" , &inh_caps, &ambient_caps));
 
 	argv += optind;
 
@@ -437,6 +471,9 @@ int setpriv_main(int argc UNUSED_PARAM, char **argv)
 #if ENABLE_FEATURE_SETPRIV_CAPABILITIES
 	if (opts & OPT_INH)
 		set_inh_caps(inh_caps);
+
+	if (opts & OPT_AMB)
+		set_ambient_caps(ambient_caps);
 #endif
 
 	if (!argv[0])
-- 
2.13.2