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Subject[PATCH 4.16 12/81] KEYS: DNS: limit the length of option strings
Date
4.16-stable review patch.  If anyone has any objections, please let me know.

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

From: Eric Biggers <ebiggers@google.com>


[ Upstream commit 9c438d7a3a52dcc2b9ed095cb87d3a5e83cf7e60 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/dns_resolver/dns_key.c | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)

--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -91,9 +91,9 @@ dns_resolver_preparse(struct key_prepars

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_prepars
}

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);
}

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