[Dnsmasq-discuss] dnsmsaq potential vulnerability

Nick Sampanis nicksampanis at gmail.com
Fri Apr 10 20:41:14 BST 2015


Your are welcome. As I stated, the return value of setup_reply will be used
as a size argument
which might be a positive 32 bit value. It seems that two things can occur
from that point.

In case of a high integer write might return -1 with errno equal to EFAULT,
in that case nothing
significant happens from attacker's perspective. Otherwise m size bytes
will be send to the attacker,
which will reveal data, allocated after packet's allocation or previously
freed data,
which has not been initialized (zeroed out)

On 9 April 2015 at 23:56, Simon Kelley <simon at thekelleys.org.uk> wrote:

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> Thanks for this. The error is obvious, and I've just committed the
> fix, to check the return value of skip_questions() in setup_reply().
>
> This is a a potential DoS attack, but I'm not clear if it's worse than
> that. The ability to read the dnsmasq heap seems to depend on details
> on the addresss-space layout over which the attacker has no control.
> (Plus, there's really not much in a dnsmasq process worth learning:
> all the data in the cache is available with a DNS query anyway!) Or am
> I being naive?
>
>
> Cheers,
>
> Simon.
>
>
>
> On 07/04/15 08:49, Nick Sampanis wrote:
> > Dear sirs, I discovered one potential vulnerability in dnsmasq.
> > More specifically, in tcp_request(), setup_reply() gets called and
> > the returned value is used as a size argument in a write function.
> >
> > m = setup_reply(header, (unsigned int)size, addrp, flags,
> > daemon->local_ttl); read_write(confd, packet, m + sizeof(u16), 0))
> >
> > Although, setup_reply can't return a size variable greater than
> > packet[65535+ MAXDNAME + RRFIXEDSZ + sizeof(u16))], an ignored
> > error value(NULL) of  skip_questions() might lead to a negative
> > pointer value(-header)
> >
> > size_t setup_reply(struct dns_header *header, size_t qlen, struct
> > all_addr *addrp, unsigned int flags, unsigned long ttl) { unsigned
> > char *p = skip_questions(header, qlen) return p - (unsigned char
> > *)header }
> >
> > read_write checks if the size argument is positive. In case of a 32
> > bit system size_t m would be 4 bytes and read_write will
> > automatically exit. In case of 64 bit system size_t m is 8 bytes
> > and may turn to positive if the sign bit of the 32 bit value is 0.
> >
> > If m is less than 0xffffffff80000000, dnsmasq will be exploited by
> > a potential attacker who will remotely read dnsmasq heap until it
> > crashes. If the above condition is not met, dnsmasq  exits
> > properly.
> >
> >
> >
> > _______________________________________________ Dnsmasq-discuss
> > mailing list Dnsmasq-discuss at lists.thekelleys.org.uk
> > http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss
> >
>
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