Subject: Re: [PATCH] KEYS: DNS: limit the length of option strings

From: Eric Biggers <[email-masked]>
Date: Wed, 28 Feb 2018 11:04:26 -0800

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 <[email-masked]>
> > 
> > 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 <[email-masked]>
> > Fixes: 4a2d789267e0 ("DNS: If the DNS server returns an error, allow that to be cached [ver #2]")
> > Cc: <[email-masked]> # v2.6.36+
> > Signed-off-by: Eric Biggers <[email-masked]>
> > ---
> >  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