KEYS: DNS: fix parsing multiple options

From: Eric Biggers <[email protected]>

My recent fix for dns_resolver_preparse() printing very long strings was incomplete, as shown by syzbot which still managed to hit the WARN_ONCE() in set_precision() by adding a crafted "dns_resolver" key:

precision 50001 too large
WARNING: CPU: 7 PID: 864 at lib/vsprintf.c:2164 vsnprintf+0x48a/0x5a0

The bug this time isn't just a printing bug, but also a logical error when multiple options ("#"-separated strings) are given in the key payload. Specifically, when separating an option string into name and value, if there is no value then the name is incorrectly considered to end at the end of the key payload, rather than the end of the current option. This bypasses validation of the option length, and also means that specifying multiple options is broken -- which presumably has gone unnoticed as there is currently only one valid option anyway.

A similar problem also applied to option values, as the kstrtoul() when parsing the "dnserror" option will read past the end of the current option and into the next option.

Fix these bugs by correctly computing the length of the option name and by copying the option value, null-terminated, into a temporary buffer.

Reproducer for the WARN_ONCE() that syzbot hit:

    perl -e 'print "#A#", "\0" x 50000' | keyctl padd dns_resolver desc @s

Reproducer for "dnserror" option being parsed incorrectly (expected behavior is to fail when seeing the unknown option "foo", actual behavior was to read the dnserror value as "1#foo" and fail there):

perl -e 'print "#dnserror=1#foo\0"' | keyctl padd dns_resolver desc @s

---

Changed since v1:
- Also fix parsing the option values, not just option names.

 net/dns_resolver/dns_key.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

index 40c851693f77..0c9478b91fa5 100644
@@ -86,35 +86,39 @@ dns_resolver_preparse(struct key_preparsed_payload *prep)
 		opt++;
 		kdebug("options: '%s'", opt);
 		do {
+			int opt_len, opt_nlen;
 			const char *eq;
-			int opt_len, opt_nlen, opt_vlen, tmp;
+			char optval[128];
 
 			next_opt = memchr(opt, '#', end - opt) ?: end;
 			opt_len = next_opt - opt;
-			if (opt_len <= 0 || opt_len > 128) {
+			if (opt_len <= 0 || opt_len > sizeof(optval)) {
 				pr_warn_ratelimited("Invalid option length (%d) for dns_resolver key\n",
 						    opt_len);
 				return -EINVAL;
 			}
 
-			eq = memchr(opt, '=', opt_len) ?: end;
-			opt_nlen = eq - opt;
-			eq++;
-			opt_vlen = next_opt - eq; /* will be -1 if no value */
+			eq = memchr(opt, '=', opt_len);
+			if (eq) {
+				opt_nlen = eq - opt;
+				eq++;
+				memcpy(optval, eq, next_opt - eq);
+				optval[next_opt - eq] = '\0';
+			} else {
+				opt_nlen = opt_len;
+				optval[0] = '\0';
+			}
 
-			tmp = opt_vlen >= 0 ? opt_vlen : 0;
-			kdebug("option '%*.*s' val '%*.*s'",
-			       opt_nlen, opt_nlen, opt, tmp, tmp, eq);
+			kdebug("option '%*.*s' val '%s'",
+			       opt_nlen, opt_nlen, opt, optval);
 
 			/* see if it's an error number representing a DNS error
 			 * that's to be recorded as the result in this key */
 			if (opt_nlen = sizeof(DNS_ERRORNO_OPTION) - 1 &&
 			    memcmp(opt, DNS_ERRORNO_OPTION, opt_nlen) = 0) {
 				kdebug("dns error number option");
-				if (opt_vlen <= 0)
-					goto bad_option_value;
 
-				ret = kstrtoul(eq, 10, &derrno);
+				ret = kstrtoul(optval, 10, &derrno);
 				if (ret < 0)
 					goto bad_option_value;
 

2.18.0.203.gfac676dfb9-goog

From: Eric Biggers <[email protected]>
Date: Wed, 11 Jul 2018 10:46:29 -0700

Applied and queued up for -stable.