[iptables,v2,2/2] iptables-restore: support acquiring the lock.

Message ID 20170316075502.2337-3-lorenzo@google.com
State Accepted
Delegated to: Pablo Neira
Headers show

Commit Message

Lorenzo Colitti March 16, 2017, 7:55 a.m.
Currently, ip[6]tables-restore does not perform any locking, so it
is not safe to use concurrently with ip[6]tables.

This patch makes ip[6]tables-restore wait for the lock if -w
was specified. Arguments to -w and -W are supported in the same
was as they are in ip[6]tables.

The lock is not acquired on startup. Instead, it is acquired when
a new table handle is created (on encountering '*') and released
when the table is committed (COMMIT). This makes it possible to
keep long-running iptables-restore processes in the background
(for example, reading commands from a pipe opened by a system
management daemon) and simultaneously run iptables commands.

If -w is not specified, then the command proceeds without taking
the lock.

Tested as follows:

1. Run iptables-restore -w, and check that iptables commands work
   with or without -w.
2. Type "*filter" into the iptables-restore input. Verify that
   a) ip[6]tables commands without -w fail with "another app is
      currently holding the xtables lock...".
   b) ip[6]tables commands with "-w 2" fail after 2 seconds.
   c) ip[6]tables commands with "-w" hang until "COMMIT" is
      typed into the iptables-restore window.
3. With the lock held by an ip6tables-restore process:
     strace -e flock /tmp/iptables/sbin/iptables-restore -w 1 -W 100000
   shows 11 calls to flock and fails.
4. Run an iptables-restore with -w and one without -w, and check:
   a) Type "*filter" in the first and then the second, and the
      second exits with an error.
   b) Type "*filter" in the second and "*filter" "-S" "COMMIT"
      into the first. The rules are listed only when the first
      copy sees "COMMIT".

Signed-off-by: Narayan Kamath <narayan@google.com>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
---
 iptables/ip6tables-restore.c | 55 ++++++++++++++++++++++++++++++++++----------
 iptables/ip6tables.c         |  2 +-
 iptables/iptables-restore.c  | 55 ++++++++++++++++++++++++++++++++++----------
 iptables/iptables.c          |  2 +-
 iptables/xshared.c           | 18 ++++++++++-----
 iptables/xshared.h           | 23 +++++++++++++++++-
 6 files changed, 122 insertions(+), 33 deletions(-)

Comments

Pablo Neira Ayuso March 17, 2017, 1:20 p.m. | #1
Hi Lorenzo,

On Thu, Mar 16, 2017 at 04:55:02PM +0900, Lorenzo Colitti wrote:
> Currently, ip[6]tables-restore does not perform any locking, so it
> is not safe to use concurrently with ip[6]tables.
> 
> This patch makes ip[6]tables-restore wait for the lock if -w
> was specified. Arguments to -w and -W are supported in the same
> was as they are in ip[6]tables.
> 
> The lock is not acquired on startup. Instead, it is acquired when
> a new table handle is created (on encountering '*') and released
> when the table is committed (COMMIT). This makes it possible to
> keep long-running iptables-restore processes in the background
> (for example, reading commands from a pipe opened by a system
> management daemon) and simultaneously run iptables commands.
> 
> If -w is not specified, then the command proceeds without taking
> the lock.
> 
> Tested as follows:
> 
> 1. Run iptables-restore -w, and check that iptables commands work
>    with or without -w.
> 2. Type "*filter" into the iptables-restore input. Verify that
>    a) ip[6]tables commands without -w fail with "another app is
>       currently holding the xtables lock...".
>    b) ip[6]tables commands with "-w 2" fail after 2 seconds.
>    c) ip[6]tables commands with "-w" hang until "COMMIT" is
>       typed into the iptables-restore window.
> 3. With the lock held by an ip6tables-restore process:
>      strace -e flock /tmp/iptables/sbin/iptables-restore -w 1 -W 100000
>    shows 11 calls to flock and fails.
> 4. Run an iptables-restore with -w and one without -w, and check:
>    a) Type "*filter" in the first and then the second, and the
>       second exits with an error.
>    b) Type "*filter" in the second and "*filter" "-S" "COMMIT"
>       into the first. The rules are listed only when the first
>       copy sees "COMMIT".

My concern with this is that one iptables-restore instance may
postpone any other iptables call indefinitely, by simply typing
"*filter" with no COMMIT ever.

The other instance can simply kill this process, as they would be both
running with CAP_NET_ADMIN though.
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Lorenzo Colitti March 17, 2017, 4:46 p.m. | #2
On Fri, Mar 17, 2017 at 10:20 PM, Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> My concern with this is that one iptables-restore instance may
> postpone any other iptables call indefinitely, by simply typing
> "*filter" with no COMMIT ever.

True. But unless the command is run with just plain "-w" (i.e., wait
forever with no timeout) then the user will get an error message that
the lock is held and can take action.

Also: are you thinking about a misconfiguration or an error in a
script, or malicious behaviour? A malicious actor can already block
any iptables commands in the same way by running "flock -e
/run/iptables.lock cat".
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Pablo Neira Ayuso March 21, 2017, 1:54 p.m. | #3
On Thu, Mar 16, 2017 at 04:55:02PM +0900, Lorenzo Colitti wrote:
> Currently, ip[6]tables-restore does not perform any locking, so it
> is not safe to use concurrently with ip[6]tables.
> 
> This patch makes ip[6]tables-restore wait for the lock if -w
> was specified. Arguments to -w and -W are supported in the same
> was as they are in ip[6]tables.
> 
> The lock is not acquired on startup. Instead, it is acquired when
> a new table handle is created (on encountering '*') and released
> when the table is committed (COMMIT). This makes it possible to
> keep long-running iptables-restore processes in the background
> (for example, reading commands from a pipe opened by a system
> management daemon) and simultaneously run iptables commands.
> 
> If -w is not specified, then the command proceeds without taking
> the lock.
> 
> Tested as follows:
> 
> 1. Run iptables-restore -w, and check that iptables commands work
>    with or without -w.
> 2. Type "*filter" into the iptables-restore input. Verify that
>    a) ip[6]tables commands without -w fail with "another app is
>       currently holding the xtables lock...".
>    b) ip[6]tables commands with "-w 2" fail after 2 seconds.
>    c) ip[6]tables commands with "-w" hang until "COMMIT" is
>       typed into the iptables-restore window.
> 3. With the lock held by an ip6tables-restore process:
>      strace -e flock /tmp/iptables/sbin/iptables-restore -w 1 -W 100000
>    shows 11 calls to flock and fails.
> 4. Run an iptables-restore with -w and one without -w, and check:
>    a) Type "*filter" in the first and then the second, and the
>       second exits with an error.
>    b) Type "*filter" in the second and "*filter" "-S" "COMMIT"
>       into the first. The rules are listed only when the first
>       copy sees "COMMIT".

Applied, thanks Lorenzo.
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Patch

diff --git a/iptables/ip6tables-restore.c b/iptables/ip6tables-restore.c
index dc0acb05a4..8a47f09c95 100644
--- a/iptables/ip6tables-restore.c
+++ b/iptables/ip6tables-restore.c
@@ -15,6 +15,7 @@ 
 #include <stdio.h>
 #include <stdlib.h>
 #include "ip6tables.h"
+#include "xshared.h"
 #include "xtables.h"
 #include "libiptc/libip6tc.h"
 #include "ip6tables-multi.h"
@@ -25,17 +26,23 @@ 
 #define DEBUGP(x, args...)
 #endif
 
-static int counters = 0, verbose = 0, noflush = 0;
+static int counters = 0, verbose = 0, noflush = 0, wait = 0;
+
+static struct timeval wait_interval = {
+	.tv_sec	= 1,
+};
 
 /* Keeping track of external matches and targets.  */
 static const struct option options[] = {
-	{.name = "counters", .has_arg = false, .val = 'c'},
-	{.name = "verbose",  .has_arg = false, .val = 'v'},
-	{.name = "test",     .has_arg = false, .val = 't'},
-	{.name = "help",     .has_arg = false, .val = 'h'},
-	{.name = "noflush",  .has_arg = false, .val = 'n'},
-	{.name = "modprobe", .has_arg = true,  .val = 'M'},
-	{.name = "table",    .has_arg = true,  .val = 'T'},
+	{.name = "counters",      .has_arg = 0, .val = 'c'},
+	{.name = "verbose",       .has_arg = 0, .val = 'v'},
+	{.name = "test",          .has_arg = 0, .val = 't'},
+	{.name = "help",          .has_arg = 0, .val = 'h'},
+	{.name = "noflush",       .has_arg = 0, .val = 'n'},
+	{.name = "modprobe",      .has_arg = 1, .val = 'M'},
+	{.name = "table",         .has_arg = 1, .val = 'T'},
+	{.name = "wait",          .has_arg = 2, .val = 'w'},
+	{.name = "wait-interval", .has_arg = 2, .val = 'W'},
 	{NULL},
 };
 
@@ -43,14 +50,16 @@  static void print_usage(const char *name, const char *version) __attribute__((no
 
 static void print_usage(const char *name, const char *version)
 {
-	fprintf(stderr, "Usage: %s [-c] [-v] [-t] [-h] [-n] [-T table] [-M command]\n"
+	fprintf(stderr, "Usage: %s [-c] [-v] [-t] [-h] [-n] [-w secs] [-W usecs] [-T table] [-M command]\n"
 			"	   [ --counters ]\n"
 			"	   [ --verbose ]\n"
 			"	   [ --test ]\n"
 			"	   [ --help ]\n"
 			"	   [ --noflush ]\n"
+			"	   [ --wait=<seconds>\n"
+			"	   [ --wait-interval=<usecs>\n"
 			"	   [ --table=<TABLE> ]\n"
-			"          [ --modprobe=<command> ]\n", name);
+			"	   [ --modprobe=<command> ]\n", name);
 
 	exit(1);
 }
@@ -181,7 +190,7 @@  int ip6tables_restore_main(int argc, char *argv[])
 {
 	struct xtc_handle *handle = NULL;
 	char buffer[10240];
-	int c;
+	int c, lock;
 	char curtable[XT_TABLE_MAXNAMELEN + 1];
 	FILE *in;
 	int in_table = 0, testing = 0;
@@ -189,6 +198,7 @@  int ip6tables_restore_main(int argc, char *argv[])
 	const struct xtc_ops *ops = &ip6tc_ops;
 
 	line = 0;
+	lock = XT_LOCK_NOT_ACQUIRED;
 
 	ip6tables_globals.program_name = "ip6tables-restore";
 	c = xtables_init_all(&ip6tables_globals, NFPROTO_IPV6);
@@ -203,7 +213,7 @@  int ip6tables_restore_main(int argc, char *argv[])
 	init_extensions6();
 #endif
 
-	while ((c = getopt_long(argc, argv, "bcvthnM:T:", options, NULL)) != -1) {
+	while ((c = getopt_long(argc, argv, "bcvthnwWM:T:", options, NULL)) != -1) {
 		switch (c) {
 			case 'b':
 				fprintf(stderr, "-b/--binary option is not implemented\n");
@@ -224,6 +234,12 @@  int ip6tables_restore_main(int argc, char *argv[])
 			case 'n':
 				noflush = 1;
 				break;
+			case 'w':
+				wait = parse_wait_time(argc, argv);
+				break;
+			case 'W':
+				parse_wait_interval(argc, argv, &wait_interval);
+				break;
 			case 'M':
 				xtables_modprobe_program = optarg;
 				break;
@@ -268,8 +284,23 @@  int ip6tables_restore_main(int argc, char *argv[])
 				DEBUGP("Not calling commit, testing\n");
 				ret = 1;
 			}
+
+			/* Done with the current table, release the lock. */
+			if (lock >= 0) {
+				xtables_unlock(lock);
+				lock = XT_LOCK_NOT_ACQUIRED;
+			}
+
 			in_table = 0;
 		} else if ((buffer[0] == '*') && (!in_table)) {
+			/* Acquire a lock before we create a new table handle */
+			lock = xtables_lock(wait, &wait_interval);
+			if (lock == XT_LOCK_BUSY) {
+				fprintf(stderr, "Another app is currently holding the xtables lock. "
+					"Perhaps you want to use the -w option?\n");
+				exit(RESOURCE_PROBLEM);
+			}
+
 			/* New table */
 			char *table;
 
diff --git a/iptables/ip6tables.c b/iptables/ip6tables.c
index 4d77721b04..579d347b09 100644
--- a/iptables/ip6tables.c
+++ b/iptables/ip6tables.c
@@ -1779,7 +1779,7 @@  int do_command6(int argc, char *argv[], char **table,
 	generic_opt_check(command, cs.options);
 
 	/* Attempt to acquire the xtables lock */
-	if (!restore && !xtables_lock(wait, &wait_interval)) {
+	if (!restore && xtables_lock(wait, &wait_interval) == XT_LOCK_BUSY) {
 		fprintf(stderr, "Another app is currently holding the xtables lock. ");
 		if (wait == 0)
 			fprintf(stderr, "Perhaps you want to use the -w option?\n");
diff --git a/iptables/iptables-restore.c b/iptables/iptables-restore.c
index 2f1522db52..7bb06d84b1 100644
--- a/iptables/iptables-restore.c
+++ b/iptables/iptables-restore.c
@@ -12,6 +12,7 @@ 
 #include <stdio.h>
 #include <stdlib.h>
 #include "iptables.h"
+#include "xshared.h"
 #include "xtables.h"
 #include "libiptc/libiptc.h"
 #include "iptables-multi.h"
@@ -22,17 +23,23 @@ 
 #define DEBUGP(x, args...)
 #endif
 
-static int counters = 0, verbose = 0, noflush = 0;
+static int counters = 0, verbose = 0, noflush = 0, wait = 0;
+
+static struct timeval wait_interval = {
+	.tv_sec	= 1,
+};
 
 /* Keeping track of external matches and targets.  */
 static const struct option options[] = {
-	{.name = "counters", .has_arg = false, .val = 'c'},
-	{.name = "verbose",  .has_arg = false, .val = 'v'},
-	{.name = "test",     .has_arg = false, .val = 't'},
-	{.name = "help",     .has_arg = false, .val = 'h'},
-	{.name = "noflush",  .has_arg = false, .val = 'n'},
-	{.name = "modprobe", .has_arg = true,  .val = 'M'},
-	{.name = "table",    .has_arg = true,  .val = 'T'},
+	{.name = "counters",      .has_arg = 0, .val = 'c'},
+	{.name = "verbose",       .has_arg = 0, .val = 'v'},
+	{.name = "test",          .has_arg = 0, .val = 't'},
+	{.name = "help",          .has_arg = 0, .val = 'h'},
+	{.name = "noflush",       .has_arg = 0, .val = 'n'},
+	{.name = "modprobe",      .has_arg = 1, .val = 'M'},
+	{.name = "table",         .has_arg = 1, .val = 'T'},
+	{.name = "wait",          .has_arg = 2, .val = 'w'},
+	{.name = "wait-interval", .has_arg = 2, .val = 'W'},
 	{NULL},
 };
 
@@ -42,14 +49,16 @@  static void print_usage(const char *name, const char *version) __attribute__((no
 
 static void print_usage(const char *name, const char *version)
 {
-	fprintf(stderr, "Usage: %s [-c] [-v] [-t] [-h] [-n] [-T table] [-M command]\n"
+	fprintf(stderr, "Usage: %s [-c] [-v] [-t] [-h] [-n] [-w secs] [-W usecs] [-T table] [-M command]\n"
 			"	   [ --counters ]\n"
 			"	   [ --verbose ]\n"
 			"	   [ --test ]\n"
 			"	   [ --help ]\n"
 			"	   [ --noflush ]\n"
+			"	   [ --wait=<seconds>\n"
+			"	   [ --wait-interval=<usecs>\n"
 			"	   [ --table=<TABLE> ]\n"
-			"          [ --modprobe=<command> ]\n", name);
+			"	   [ --modprobe=<command> ]\n", name);
 
 	exit(1);
 }
@@ -180,7 +189,7 @@  iptables_restore_main(int argc, char *argv[])
 {
 	struct xtc_handle *handle = NULL;
 	char buffer[10240];
-	int c;
+	int c, lock;
 	char curtable[XT_TABLE_MAXNAMELEN + 1];
 	FILE *in;
 	int in_table = 0, testing = 0;
@@ -188,6 +197,7 @@  iptables_restore_main(int argc, char *argv[])
 	const struct xtc_ops *ops = &iptc_ops;
 
 	line = 0;
+	lock = XT_LOCK_NOT_ACQUIRED;
 
 	iptables_globals.program_name = "iptables-restore";
 	c = xtables_init_all(&iptables_globals, NFPROTO_IPV4);
@@ -202,7 +212,7 @@  iptables_restore_main(int argc, char *argv[])
 	init_extensions4();
 #endif
 
-	while ((c = getopt_long(argc, argv, "bcvthnM:T:", options, NULL)) != -1) {
+	while ((c = getopt_long(argc, argv, "bcvthnwWM:T:", options, NULL)) != -1) {
 		switch (c) {
 			case 'b':
 				fprintf(stderr, "-b/--binary option is not implemented\n");
@@ -223,6 +233,12 @@  iptables_restore_main(int argc, char *argv[])
 			case 'n':
 				noflush = 1;
 				break;
+			case 'w':
+				wait = parse_wait_time(argc, argv);
+				break;
+			case 'W':
+				parse_wait_interval(argc, argv, &wait_interval);
+				break;
 			case 'M':
 				xtables_modprobe_program = optarg;
 				break;
@@ -267,8 +283,23 @@  iptables_restore_main(int argc, char *argv[])
 				DEBUGP("Not calling commit, testing\n");
 				ret = 1;
 			}
+
+			/* Done with the current table, release the lock. */
+			if (lock >= 0) {
+				xtables_unlock(lock);
+				lock = XT_LOCK_NOT_ACQUIRED;
+			}
+
 			in_table = 0;
 		} else if ((buffer[0] == '*') && (!in_table)) {
+			/* Acquire a lock before we create a new table handle */
+			lock = xtables_lock(wait, &wait_interval);
+			if (lock == XT_LOCK_BUSY) {
+				fprintf(stderr, "Another app is currently holding the xtables lock. "
+					"Perhaps you want to use the -w option?\n");
+				exit(RESOURCE_PROBLEM);
+			}
+
 			/* New table */
 			char *table;
 
diff --git a/iptables/iptables.c b/iptables/iptables.c
index 04be5abb10..62731c5ebb 100644
--- a/iptables/iptables.c
+++ b/iptables/iptables.c
@@ -1766,7 +1766,7 @@  int do_command4(int argc, char *argv[], char **table,
 	generic_opt_check(command, cs.options);
 
 	/* Attempt to acquire the xtables lock */
-	if (!restore && !xtables_lock(wait, &wait_interval)) {
+	if (!restore && xtables_lock(wait, &wait_interval) == XT_LOCK_BUSY) {
 		fprintf(stderr, "Another app is currently holding the xtables lock. ");
 		if (wait == 0)
 			fprintf(stderr, "Perhaps you want to use the -w option?\n");
diff --git a/iptables/xshared.c b/iptables/xshared.c
index dd5f8be028..5a7fcc0046 100644
--- a/iptables/xshared.c
+++ b/iptables/xshared.c
@@ -245,7 +245,7 @@  void xs_init_match(struct xtables_match *match)
 		match->init(match->m);
 }
 
-bool xtables_lock(int wait, struct timeval *wait_interval)
+int xtables_lock(int wait, struct timeval *wait_interval)
 {
 	struct timeval time_left, wait_time;
 	int fd, i = 0;
@@ -255,22 +255,22 @@  bool xtables_lock(int wait, struct timeval *wait_interval)
 
 	fd = open(XT_LOCK_NAME, O_CREAT, 0600);
 	if (fd < 0)
-		return true;
+		return XT_LOCK_UNSUPPORTED;
 
 	if (wait == -1) {
 		if (flock(fd, LOCK_EX) == 0)
-			return true;
+			return fd;
 
 		fprintf(stderr, "Can't lock %s: %s\n", XT_LOCK_NAME,
 			strerror(errno));
-		return false;
+		return XT_LOCK_BUSY;
 	}
 
 	while (1) {
 		if (flock(fd, LOCK_EX | LOCK_NB) == 0)
-			return true;
+			return fd;
 		else if (timercmp(&time_left, wait_interval, <))
-			return false;
+			return XT_LOCK_BUSY;
 
 		if (++i % 10 == 0) {
 			fprintf(stderr, "Another app is currently holding the xtables lock; "
@@ -284,6 +284,12 @@  bool xtables_lock(int wait, struct timeval *wait_interval)
 	}
 }
 
+void xtables_unlock(int lock)
+{
+	if (lock >= 0)
+		close(lock);
+}
+
 int parse_wait_time(int argc, char *argv[])
 {
 	int wait = -1;
diff --git a/iptables/xshared.h b/iptables/xshared.h
index d8a697ae66..539e6c243b 100644
--- a/iptables/xshared.h
+++ b/iptables/xshared.h
@@ -86,7 +86,28 @@  extern struct xtables_match *load_proto(struct iptables_command_state *);
 extern int subcmd_main(int, char **, const struct subcommand *);
 extern void xs_init_target(struct xtables_target *);
 extern void xs_init_match(struct xtables_match *);
-bool xtables_lock(int wait, struct timeval *wait_interval);
+
+/**
+ * Values for the iptables lock.
+ *
+ * A value >= 0 indicates the lock filedescriptor. Other values are:
+ *
+ * XT_LOCK_UNSUPPORTED : The system does not support locking, execution will
+ * proceed lockless.
+ *
+ * XT_LOCK_BUSY : The lock was held by another process. xtables_lock only
+ * returns this value when |wait| == false. If |wait| == true, xtables_lock
+ * will not return unless the lock has been acquired.
+ *
+ * XT_LOCK_NOT_ACQUIRED : We have not yet attempted to acquire the lock.
+ */
+enum {
+	XT_LOCK_BUSY = -1,
+	XT_LOCK_UNSUPPORTED  = -2,
+	XT_LOCK_NOT_ACQUIRED  = -3,
+};
+extern int xtables_lock(int wait, struct timeval *tv);
+extern void xtables_unlock(int lock);
 
 int parse_wait_time(int argc, char *argv[]);
 void parse_wait_interval(int argc, char *argv[], struct timeval *wait_interval);