From: Eric Biggers <ebiggers@google.com>
Date: Tue, 17 Apr 2018 12:07:06 -0700
Subject: KEYS: DNS: limit the length of option strings
Git-commit: 9c438d7a3a52dcc2b9ed095cb87d3a5e83cf7e60
No-fix: c210f7b411790ee0fc4252547d645c5d8648adc5
Patch-mainline: v4.17-rc2
References: networking-stable-18_04_26

Adding a dns_resolver key whose payload contains a very long option name
resulted in that string being printed in full.  This hit the WARN_ONCE()
in set_precision() during the printk(), because printk() only supports a
precision of up to 32767 bytes:

    precision 1000000 too large
    WARNING: CPU: 0 PID: 752 at lib/vsprintf.c:2189 vsnprintf+0x4bc/0x5b0

Fix it by limiting option strings (combined name + value) to a much more
reasonable 128 bytes.  The exact limit is arbitrary, but currently the
only recognized option is formatted as "dnserror=%lu" which fits well
within this limit.

Also ratelimit the printks.

Reproducer:

    perl -e 'print "#", "A" x 1000000, "\x00"' | keyctl padd dns_resolver desc @s

This bug was found using syzkaller.

[js] add ratelimit.h include

Reported-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 4a2d789267e0 ("DNS: If the DNS server returns an error, allow that to be cached [ver #2]")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 net/dns_resolver/dns_key.c |   13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -28,6 +28,7 @@
 #include <linux/kernel.h>
 #include <linux/keyctl.h>
 #include <linux/err.h>
+#include <linux/ratelimit.h>
 #include <linux/seq_file.h>
 #include <keys/dns_resolver-type.h>
 #include <keys/user-type.h>
@@ -91,9 +92,9 @@ dns_resolver_preparse(struct key_prepars
 
 			next_opt = memchr(opt, '#', end - opt) ?: end;
 			opt_len = next_opt - opt;
-			if (!opt_len) {
-				printk(KERN_WARNING
-				       "Empty option to dns_resolver key\n");
+			if (opt_len <= 0 || opt_len > 128) {
+				pr_warn_ratelimited("Invalid option length (%d) for dns_resolver key\n",
+						    opt_len);
 				return -EINVAL;
 			}
 
@@ -127,10 +128,8 @@ dns_resolver_preparse(struct key_prepars
 			}
 
 		bad_option_value:
-			printk(KERN_WARNING
-			       "Option '%*.*s' to dns_resolver key:"
-			       " bad/missing value\n",
-			       opt_nlen, opt_nlen, opt);
+			pr_warn_ratelimited("Option '%*.*s' to dns_resolver key: bad/missing value\n",
+					    opt_nlen, opt_nlen, opt);
 			return -EINVAL;
 		} while (opt = next_opt + 1, opt < end);
 	}