KEYS: DNS: limit the length of option strings

commit 9c438d7a3a52dcc2b9ed095cb87d3a5e83cf7e60 upstream.

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.

Change-Id: I5e22ee8ad8da3cc222e35239f05591c893d3f508
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>
[bwh: Backported to 3.16:
 - Also stop logging the key serial number
 - Include <linux/ratelimit.h> directly]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

1 files changed, 6 insertions, 8 deletions

diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
index 0a69d0757..cb88491d3 100644
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -30,6 +30,7 @@
 #include <linux/keyctl.h>
 #include <linux/err.h>
 #include <linux/seq_file.h>
+#include <linux/ratelimit.h>
 #include <keys/dns_resolver-type.h>
 #include <keys/user-type.h>
 #include "internal.h"
@@ -94,10 +95,9 @@ dns_resolver_instantiate(struct key *key, struct key_preparsed_payload *prep)
 
 			next_opt = memchr(opt, '#', end - opt) ?: end;
 			opt_len = next_opt - opt;
-			if (!opt_len) {
-				printk(KERN_WARNING
-				       "Empty option to dns_resolver key %d\n",
-				       key->serial);
+			if (opt_len <= 0 || opt_len > 128) {
+				pr_warn_ratelimited("Invalid option length (%d) for dns_resolver key\n",
+						    opt_len);
 				return -EINVAL;
 			}
 
@@ -131,10 +131,8 @@ dns_resolver_instantiate(struct key *key, struct key_preparsed_payload *prep)
 			}
 
 		bad_option_value:
-			printk(KERN_WARNING
-			       "Option '%*.*s' to dns_resolver key %d:"
-			       " bad/missing value\n",
-			       opt_nlen, opt_nlen, opt, key->serial);
+			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);
 	}