xshared: Fix response to unprivileged users

Expected behaviour in both variants is:

* Print help without error, append extension help if -m and/or -j
  options are present
* Indicate lack of permissions in an error message for anything else

With iptables-nft, this was broken basically from day 1. Shared use of
do_parse() then somewhat broke legacy: it started complaining about
inability to create a lock file.

Fix this by making iptables-nft assume extension revision 0 is present
if permissions don't allow to verify. This is consistent with legacy.

Second part is to exit directly after printing help - this avoids having
to make the following code "nop-aware" to prevent privileged actions.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Florian Westphal <fw@strlen.de>

3 files changed, 66 insertions, 2 deletions

diff --git a/iptables/nft.c b/iptables/nft.c
index 72f7cf13..b5de687c 100644
--- a/iptables/nft.c
+++ b/iptables/nft.c
@@ -3312,6 +3312,11 @@ int nft_compatible_revision(const char *name, uint8_t rev, int opt)
 err:
 	mnl_socket_close(nl);
 
+	/* pretend revision 0 is valid if not permitted to check -
+	 * this is required for printing extension help texts as user */
+	if (ret < 0 && errno == EPERM && rev == 0)
+		return 1;
+
 	return ret < 0 ? 0 : 1;
 }
 
diff --git a/iptables/tests/shell/testcases/iptables/0008-unprivileged_0 b/iptables/tests/shell/testcases/iptables/0008-unprivileged_0
new file mode 100755
index 00000000..43e3bc87
--- /dev/null
+++ b/iptables/tests/shell/testcases/iptables/0008-unprivileged_0
@@ -0,0 +1,60 @@
+#!/bin/bash
+
+# iptables may print match/target specific help texts
+# help output should work for unprivileged users
+
+run() {
+	echo "running: $*" >&2
+	runuser -u nobody -- "$@"
+}
+
+grep_or_rc() {
+	declare -g rc
+	grep -q "$*" && return 0
+	echo "missing in output: $*" >&2
+	return 1
+}
+
+out=$(run $XT_MULTI iptables --help)
+let "rc+=$?"
+grep_or_rc "iptables -h (print this help information)" <<< "$out"
+let "rc+=$?"
+
+out=$(run $XT_MULTI iptables -m limit --help)
+let "rc+=$?"
+grep_or_rc "limit match options:" <<< "$out"
+let "rc+=$?"
+
+out=$(run $XT_MULTI iptables -p tcp --help)
+let "rc+=$?"
+grep_or_rc "tcp match options:" <<< "$out"
+let "rc+=$?"
+
+out=$(run $XT_MULTI iptables -j DNAT --help)
+let "rc+=$?"
+grep_or_rc "DNAT target options:" <<< "$out"
+let "rc+=$?"
+
+out=$(run $XT_MULTI iptables -p tcp -j DNAT --help)
+let "rc+=$?"
+grep_or_rc "tcp match options:" <<< "$out"
+let "rc+=$?"
+out=$(run $XT_MULTI iptables -p tcp -j DNAT --help)
+let "rc+=$?"
+grep_or_rc "DNAT target options:" <<< "$out"
+let "rc+=$?"
+
+
+run $XT_MULTI iptables -L 2>&1 | \
+	grep_or_rc "Permission denied"
+let "rc+=$?"
+
+run $XT_MULTI iptables -A FORWARD -p tcp --dport 123 2>&1 | \
+	grep_or_rc "Permission denied"
+let "rc+=$?"
+
+run $XT_MULTI iptables -A FORWARD -j DNAT --to-destination 1.2.3.4 2>&1 | \
+	grep_or_rc "Permission denied"
+let "rc+=$?"
+
+exit $rc
diff --git a/iptables/xshared.c b/iptables/xshared.c
index c5a93290..1fd7acc9 100644
--- a/iptables/xshared.c
+++ b/iptables/xshared.c
@@ -1473,8 +1473,7 @@ void do_parse(int argc, char *argv[],
 					XTF_TRY_LOAD, &cs->matches);
 
 			xt_params->print_help(cs->matches);
-			p->command = CMD_NONE;
-			return;
+			exit(0);
 
 			/*
 			 * Option selection