On Tue, 2018-02-27 at 17:49 -0800, Eric Biggers wrote:
> From: Eric Biggers <ebigg...@google.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.
>
> 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.rutl...@arm.com>
> Fixes: 4a2d789267e0 ("DNS: If the DNS server returns an error, allow that to
> be cached [ver #2]")
> Cc: <sta...@vger.kernel.org> # v2.6.36+
> Signed-off-by: Eric Biggers <ebigg...@google.com>
> ---
> net/dns_resolver/dns_key.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
> index e1d4d898a007..7c0aae2e512d 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("Invalid option length (%d) for
> dns_resolver key\n",
> + opt_len);
If a bot can harass us here, then pr_warn_ratelimited would be nice ?
