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

Ben Hutchings
2018-10-14T23:12:14Z

From: Eric Biggers <ebiggers@google.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@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>
---
--- 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);
}