linux-kernel - [PATCH 3.16 168/366] KEYS: DNS: limit the length of option strings

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Message-ID: <lsq.1539530741.188282517@decadent.org.uk>
Date:   Sun, 14 Oct 2018 16:25:41 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Eric Biggers" <ebiggers@...gle.com>,
        "Mark Rutland" <mark.rutland@....com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 3.16 168/366] KEYS: DNS: limit the length of option strings

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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

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.

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>
Signed-off-by: David S. Miller <davem@...emloft.net>
[bwh: Backported to 3.16:
 - Also stop logging the key serial number
 - Include <linux/ratelimit.h> directly]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -29,6 +29,7 @@
 #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"
@@ -93,10 +94,9 @@ dns_resolver_instantiate(struct key *key
 
 			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;
 			}
 
@@ -130,10 +130,8 @@ dns_resolver_instantiate(struct key *key
 			}
 
 		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);
 	}