netdev - [PATCH RESEND net v2] KEYS: DNS: limit the length of option strings

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Message-Id: <20180417190706.217384-1-ebiggers3@gmail.com>
Date:   Tue, 17 Apr 2018 12:07:06 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>
Cc:     keyrings@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: [PATCH RESEND net v2] KEYS: DNS: limit the length of option strings

From: Eric Biggers <ebiggers@...gle.com>

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@....com>
Fixes: 4a2d789267e0 ("DNS: If the DNS server returns an error, allow that to be cached [ver #2]")
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 net/dns_resolver/dns_key.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
index 8396705deffc..40c851693f77 100644
--- 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);
 	}
-- 
2.17.0.484.g0c8726318c-goog