KEYS: DNS: limit the length of option strings

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.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 492a2dbc9b95 ("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>

diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
index 8396705deffc965ba8ab58aae8ee519f3d441a64..40c851693f77e35a1f573fdbf0bcd86adb94cf13 100644 (file)
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -91,9 +91,9 @@ dns_resolver_preparse(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\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 +127,8 @@ dns_resolver_preparse(struct key_preparsed_payload *prep)
                        }
 
                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);
        }