Merge branch 'SYSRQ'

extensions/libxt_SYSRQ.man

@@ -1,17 +1,19 @@
 The SYSRQ target allows to remotely trigger sysrq on the local machine over the
 network. This can be useful when vital parts of the machine hang, for example
 an oops in a filesystem causing locks to be not released and processes to get
-stuck as a result -- if still possible, use /proc/sysrq-trigger. Even when
+stuck as a result - if still possible, use /proc/sysrq-trigger. Even when
 processes are stuck, interrupts are likely to be still processed, and as such,
 sysrq can be triggered through incoming network packets.
 .PP
-This xt_SYSRQ implementation does not use any encryption, so you should change
+The xt_SYSRQ implementation uses a salted hash and a sequence number to prevent
-the SYSRQ password after use unless you have made sure it was transmitted
+network sniffers from either guessing the password or replaying earlier
-securely and no one sniffed the network, e.g. by use of an IPsec tunnel whose
+requests. The initial sequence number comes from the time of day so you will
-endpoint is at the machine where you want to trigger the sysrq. Also, you
+have a small window of vulnerability should time go backwards at a reboot.
-should limit as to who can issue commands using \fB-s\fP and/or \fB-m mac\fP,
+However, the file /sys/module/xt_SYSREQ/seqno can be used to both query and
-and also that the destination is correct using \fB-d\fP (to protect against
+update the current sequence number. Also, you should limit as to who can issue
-potential broadcast packets), noting that it is still short of MAC/IP spoofing:
+commands using \fB-s\fP and/or \fB-m mac\fP, and also that the destination is
+correct using \fB-d\fP (to protect against potential broadcast packets), noting
+that it is still short of MAC/IP spoofing:
 .IP
 -A INPUT -s 10.10.25.1 -m mac --mac-source aa:bb:cc:dd:ee:ff -d 10.10.25.7
 -p udp --dport 9 -j SYSRQ
@@ -20,28 +22,59 @@ potential broadcast packets), noting that it is still short of MAC/IP spoofing:
 ipsec --proto esp --tunnel-src 10.10.25.1 --tunnel-dst 10.10.25.7
 -p udp --dport 9 -j SYSRQ
 .PP
+You should also limit the rate at which connections can be received to limit
+the CPU time taken by illegal requests, for example:
+.IP
+-A INPUT 0s 10.10.25.1 -m mac --mac-source aa:bb:cc:dd:ee:ff -d 10.10.25.7
+-p udp --dport 9 -m limit --limit 5/minute -j SYSRQ
+.PP
 This extension does not take any options. The \fB-p udp\fP options are
 required.
 .PP
 The SYSRQ password can be changed through
-/sys/module/xt_SYSRQ/parameters/password; note you need to use `echo -n` to
+/sys/module/xt_SYSRQ/parameters/password, for example:
-not add a newline to the password, i.e.
 .IP
-echo -n "password" >/sys/.../password
+echo -n "password" >/sys/module/xt_SYSRQ/parameters/password
 .PP
 Alternatively, the password may be specified at modprobe time, but this is
 insecure as people can possible see it through ps(1). You can use an option
-line in /etc/modprobe.d/sysrq if it is properly guarded, that is, only readable
+line in e.g. /etc/modprobe.d/xt_sysrq if it is properly guarded, that is, only
-by root.
+readable by root.
 .IP
 options xt_SYSRQ password=cookies
 .PP
-To trigger SYSRQ from a remote host, just use netcat or socat, specifying the
+The hash algorithm can also be specified as a module option, for example, to
-action (only one) as first character, followed by the password:
+use SHA-256 instead of the default SHA-1:
 .IP
-echo -n "scookies" | socat stdin udp-sendto:10.10.25.7:9
+options xt_SYSRQ hash=sha256
-.IP
-echo -n "scookies" | netcat -u 10.10.25.7 9
 .PP
-See the Linux docs for possible sysrq keys. Important ones are:
+The xt_SYSRQ module is normally silent unless a successful request is received,
-re(b)oot, power(o)ff, (s)ync filesystems, (u)mount and remount readonly.
+but the \fIdebug\fP module parameter can be used to find exactly why a
+seemingly correct request is not being processed.
+.PP
+To trigger SYSRQ from a remote host, just use netcat or socat:
+.PP
+.nf
+sysrq_key="s"  # the SysRq key(s)
+password="password"
+seqno="$(date +%s)"
+salt="$(dd bs=12 count=1 if=/dev/urandom 2>/dev/null |
+    openssl enc -base64)"
+req="$sysrq_key,$seqno,$salt"
+req="$req,$(echo -n "$req,$password" | sha1sum | cut -c1-40)"
+
+echo "$req" | socat stdin udp-sendto:10.10.25.7:9
+# or
+echo "$req" | netcat -uw1 10.10.25.7 9
+.fi
+.PP
+See the Linux docs for possible sysrq keys. Important ones are: re(b)oot,
+power(o)ff, (s)ync filesystems, (u)mount and remount readonly.  More than one
+sysrq key can be used at once, but bear in mind that, for example, a sync may
+not complete before a subsequent reboot or poweroff.
+.PP
+The hashing scheme should be enough to prevent mis-use of SYSRQ in many
+environments, but it is not perfect: take reasonable precautions to
+protect your machines.  Most importantly ensure that each machine has a
+different password; there is scant protection for a SYSRQ packet being
+applied to a machine that happens to have the same password.

extensions/xt_SYSRQ.c

@@ -3,7 +3,6 @@
  *	Copyright © Jan Engelhardt <jengelh [at] medozas de>, 2008
  *
  *	Based upon the ipt_SYSRQ idea by Marek Zalem <marek [at] terminus sk>
- *	xt_SYSRQ does not use hashing or timestamps.
  *
  *	This program is free software; you can redistribute it and/or
  *	modify it under the terms of the GNU General Public License
@@ -19,15 +18,145 @@
 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <linux/netfilter_ipv6/ip6_tables.h>
 #include <linux/netfilter/x_tables.h>
+#include <linux/crypto.h>
+#include <linux/scatterlist.h>
 #include <net/ip.h>
 #include "compat_xtables.h"
 
 static bool sysrq_once;
 static char sysrq_password[64];
+static char sysrq_hash[16] = "sha1";
+static long sysrq_seqno;
+static int sysrq_debug;
 module_param_string(password, sysrq_password, sizeof(sysrq_password),
 	S_IRUSR | S_IWUSR);
+module_param_string(hash, sysrq_hash, sizeof(sysrq_hash), S_IRUSR);
+module_param_named(seqno, sysrq_seqno, long, S_IRUSR | S_IWUSR);
+module_param_named(debug, sysrq_debug, int, S_IRUSR | S_IWUSR);
 MODULE_PARM_DESC(password, "password for remote sysrq");
+MODULE_PARM_DESC(hash, "hash algorithm, default sha1");
+MODULE_PARM_DESC(seqno, "sequence number for remote sysrq");
+MODULE_PARM_DESC(debug, "debugging: 0=off, 1=on");
 
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 19)
+static struct crypto_hash *sysrq_tfm;
+static int sysrq_digest_size;
+static unsigned char *sysrq_digest_password;
+static unsigned char *sysrq_digest;
+static char *sysrq_hexdigest;
+
+/*
+ * The data is of the form "<requests>,<seqno>,<salt>,<hash>" where <requests>
+ * is a series of sysrq requests; <seqno> is a sequence number that must be
+ * greater than the last sequence number; <salt> is some random bytes; and
+ * <hash> is the hash of everything up to and including the preceding ","
+ * together with the password.
+ *
+ * For example
+ *
+ *   salt=$RANDOM
+ *   req="s,$(date +%s),$salt"
+ *   echo "$req,$(echo -n $req,secret | sha1sum | cut -c1-40)"
+ *
+ * You will want a better salt and password than that though :-)
+ */
+static unsigned int sysrq_tg(const void *pdata, uint16_t len)
+{
+	const char *data = pdata;
+	int i, n;
+	struct scatterlist sg[2];
+	struct hash_desc desc;
+	int ret;
+	long new_seqno = 0;
+
+	if (*sysrq_password == '\0') {
+		if (!sysrq_once)
+			printk(KERN_INFO KBUILD_MODNAME ": No password set\n");
+		sysrq_once = true;
+		return NF_DROP;
+	}
+	if (len == 0)
+		return NF_DROP;
+
+	for (i = 0; sysrq_password[i] != '\0' &&
+	     sysrq_password[i] != '\n'; ++i)
+		/* loop */;
+	sysrq_password[i] = '\0';
+
+	i = 0;
+	for (n = 0; n < len - 1; ++n) {
+		if (i == 1 && '0' <= data[n] && data[n] <= '9')
+			new_seqno = 10L * new_seqno + data[n] - '0';
+		if (data[n] == ',' && ++i == 3)
+			break;
+	}
+	++n;
+	if (i != 3) {
+		if (sysrq_debug)
+			printk(KERN_WARNING KBUILD_MODNAME
+				": badly formatted request\n");
+		return NF_DROP;
+	}
+	if (sysrq_seqno >= new_seqno) {
+		if (sysrq_debug)
+			printk(KERN_WARNING KBUILD_MODNAME
+				": old sequence number ignored\n");
+		return NF_DROP;
+	}
+
+	desc.tfm   = sysrq_tfm;
+	desc.flags = 0;
+	ret = crypto_hash_init(&desc);
+	if (ret != 0)
+		goto hash_fail;
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 24)
+	sg_init_table(sg, 2);
+#endif
+	sg_set_buf(&sg[0], data, n);
+	strcpy(sysrq_digest_password, sysrq_password);
+	i = strlen(sysrq_digest_password);
+	sg_set_buf(&sg[1], sysrq_digest_password, i);
+	ret = crypto_hash_digest(&desc, sg, n + i, sysrq_digest);
+	if (ret != 0)
+		goto hash_fail;
+
+	for (i = 0; i < sysrq_digest_size; ++i) {
+		sysrq_hexdigest[2*i] =
+			"0123456789abcdef"[(sysrq_digest[i] >> 4) & 0xf];
+		sysrq_hexdigest[2*i+1] =
+			"0123456789abcdef"[sysrq_digest[i] & 0xf];
+	}
+	sysrq_hexdigest[2*sysrq_digest_size] = '\0';
+	if (len - n < sysrq_digest_size) {
+		if (sysrq_debug)
+			printk(KERN_INFO KBUILD_MODNAME ": Short digest,"
+			       " expected %s\n", sysrq_hexdigest);
+		return NF_DROP;
+	}
+	if (strncmp(data + n, sysrq_hexdigest, sysrq_digest_size) != 0) {
+		if (sysrq_debug)
+			printk(KERN_INFO KBUILD_MODNAME ": Bad digest,"
+			       " expected %s\n", sysrq_hexdigest);
+		return NF_DROP;
+	}
+
+	/* Now we trust the requester */
+	sysrq_seqno = new_seqno;
+	for (i = 0; i < len && data[i] != ','; ++i) {
+		printk(KERN_INFO KBUILD_MODNAME ": SysRq %c\n", data[i]);
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 19)
+		handle_sysrq(data[i], NULL);
+#else
+		handle_sysrq(data[i], NULL, NULL);
+#endif
+	}
+	return NF_ACCEPT;
+
+ hash_fail:
+	printk(KERN_WARNING KBUILD_MODNAME ": digest failure\n");
+	return NF_DROP;
+}
+#else
 static unsigned int sysrq_tg(const void *pdata, uint16_t len)
 {
 	const char *data = pdata;
@@ -57,6 +186,7 @@ static unsigned int sysrq_tg(const void *pdata, uint16_t len)
 #endif
 	return NF_ACCEPT;
 }
+#endif
 
 static unsigned int
 sysrq_tg4(struct sk_buff **pskb, const struct xt_target_param *par)
@@ -73,9 +203,11 @@ sysrq_tg4(struct sk_buff **pskb, const struct xt_target_param *par)
 	udph = (void *)iph + ip_hdrlen(skb);
 	len  = ntohs(udph->len) - sizeof(struct udphdr);
 
-	printk(KERN_INFO KBUILD_MODNAME ": " NIPQUAD_FMT ":%u -> :%u len=%u\n",
+	if (sysrq_debug)
-	       NIPQUAD(iph->saddr), htons(udph->source), htons(udph->dest),
+		printk(KERN_INFO KBUILD_MODNAME
-	       len);
+		       ": " NIPQUAD_FMT ":%u -> :%u len=%u\n",
+		       NIPQUAD(iph->saddr), htons(udph->source),
+		       htons(udph->dest), len);
 	return sysrq_tg((void *)udph + sizeof(struct udphdr), len);
 }
 
@@ -94,14 +226,17 @@ sysrq_tg6(struct sk_buff **pskb, const struct xt_target_param *par)
 	udph = udp_hdr(skb);
 	len  = ntohs(udph->len) - sizeof(struct udphdr);
 
-	printk(KERN_INFO KBUILD_MODNAME ": " NIP6_FMT ":%hu -> :%hu len=%u\n",
+	if (sysrq_debug)
-	       NIP6(iph->saddr), ntohs(udph->source),
+		printk(KERN_INFO KBUILD_MODNAME
-	       ntohs(udph->dest), len);
+		       ": " NIP6_FMT ":%hu -> :%hu len=%u\n",
+		       NIP6(iph->saddr), ntohs(udph->source),
+		       ntohs(udph->dest), len);
 	return sysrq_tg(udph + sizeof(struct udphdr), len);
 }
 
 static bool sysrq_tg_check(const struct xt_tgchk_param *par)
 {
+
 	if (par->target->family == NFPROTO_IPV4) {
 		const struct ipt_entry *entry = par->entryinfo;
 
@@ -146,11 +281,64 @@ static struct xt_target sysrq_tg_reg[] __read_mostly = {
 
 static int __init sysrq_tg_init(void)
 {
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 19)
+	struct timeval now;
+
+	sysrq_tfm = crypto_alloc_hash(sysrq_hash, 0, CRYPTO_ALG_ASYNC);
+	if (IS_ERR(sysrq_tfm)) {
+		printk(KERN_WARNING KBUILD_MODNAME
+			": Error: Could not find or load %s hash\n",
+			sysrq_hash);
+		sysrq_tfm = NULL;
+		goto fail;
+	}
+	sysrq_digest_size = crypto_hash_digestsize(sysrq_tfm);
+	sysrq_digest = kmalloc(sysrq_digest_size, GFP_KERNEL);
+	if (sysrq_digest == NULL) {
+		printk(KERN_WARNING KBUILD_MODNAME
+			": Cannot allocate digest\n");
+		goto fail;
+	}
+	sysrq_hexdigest = kmalloc(2 * sysrq_digest_size + 1, GFP_KERNEL);
+	if (sysrq_hexdigest == NULL) {
+		printk(KERN_WARNING KBUILD_MODNAME
+			": Cannot allocate hexdigest\n");
+		goto fail;
+	}
+	sysrq_digest_password = kmalloc(sizeof(sysrq_password), GFP_KERNEL);
+	if (sysrq_digest_password == NULL) {
+		printk(KERN_WARNING KBUILD_MODNAME
+			": Cannot allocate password digest space\n");
+		goto fail;
+	}
+	do_gettimeofday(&now);
+	sysrq_seqno = now.tv_sec;
 	return xt_register_targets(sysrq_tg_reg, ARRAY_SIZE(sysrq_tg_reg));
+
+ fail:
+	if (sysrq_tfm)
+		crypto_free_hash(sysrq_tfm);
+	if (sysrq_digest)
+		kfree(sysrq_digest);
+	if (sysrq_hexdigest)
+		kfree(sysrq_hexdigest);
+	if (sysrq_digest_password)
+		kfree(sysrq_digest_password);
+	return -EINVAL;
+#else
+	printk(KERN_WARNING "xt_SYSRQ does not provide crypto for <= 2.6.18\n");
+	return xt_register_targets(sysrq_tg_reg, ARRAY_SIZE(sysrq_tg_reg));
+#endif
 }
 
 static void __exit sysrq_tg_exit(void)
 {
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 19)
+	crypto_free_hash(sysrq_tfm);
+	kfree(sysrq_digest);
+	kfree(sysrq_hexdigest);
+	kfree(sysrq_digest_password);
+#endif
 	return xt_unregister_targets(sysrq_tg_reg, ARRAY_SIZE(sysrq_tg_reg));
 }