On Tue, Feb 27, 2018 at 06:34:19PM -0800, Eric Dumazet wrote:
> 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 ?
>
I guess we might as well. But there is another user-triggerable printk just
below, so I'll need to change that one too. I'll send out v2.
Eric
