On Mon, Jun 11, 2018 at 10:57:42AM -0700, Eric Biggers wrote:
> Hi Simon,
>
> On Mon, Jun 11, 2018 at 11:40:23AM +0200, Simon Horman wrote:
> > On Fri, Jun 08, 2018 at 09:20:37AM -0700, Eric Biggers wrote:
> > > From: Eric Biggers <ebigg...@google.com>
> > >
> > > My recent fix for dns_resolver_preparse() printing very long strings was
> > > incomplete, as shown by syzbot which still managed to hit the
> > > WARN_ONCE() in set_precision() by adding a crafted "dns_resolver" key:
> > >
> > > precision 50001 too large
> > > WARNING: CPU: 7 PID: 864 at lib/vsprintf.c:2164 vsnprintf+0x48a/0x5a0
> > >
> > > The bug this time isn't just a printing bug, but also a logical error
> > > when multiple options ("#"-separated strings) are given in the key
> > > payload. Specifically, when separating an option string into name and
> > > value, if there is no value then the name is incorrectly considered to
> > > end at the end of the key payload, rather than the end of the current
> > > option. This bypasses validation of the option length, and also means
> > > that specifying multiple options is broken -- which presumably has gone
> > > unnoticed as there is currently only one valid option anyway.
> > >
> > > Fix it by correctly calculating the length of the option name.
> > >
> > > Reproducer:
> > >
> > > perl -e 'print "#A#", "\x00" x 50000' | keyctl padd dns_resolver desc
> > > @s
> > >
> > > Fixes: 4a2d789267e0 ("DNS: If the DNS server returns an error, allow that
> > > to be cached [ver #2]")
> > > Signed-off-by: Eric Biggers <ebigg...@google.com>
> > > ---
> > > net/dns_resolver/dns_key.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
> > > index 40c851693f77e..d448823d4d2ed 100644
> > > --- a/net/dns_resolver/dns_key.c
> > > +++ b/net/dns_resolver/dns_key.c
> > > @@ -97,7 +97,7 @@ dns_resolver_preparse(struct key_preparsed_payload
> > > *prep)
> > > return -EINVAL;
> > > }
> > >
> > > - eq = memchr(opt, '=', opt_len) ?: end;
> > > + eq = memchr(opt, '=', opt_len) ?: next_opt;
> > > opt_nlen = eq - opt;
> > > eq++;
> >
> > It seems risky to advance eq++ in the case there the value is empty.
> > Its not not pointing to the value but it may be accessed twice further on
> > in this loop.
> >
>
> Sure, that's a separate existing issue though, and it must be checked that the
> value is present before using it anyway, which the code already does, so it's
> not a "real" bug. I think I'll keep this patch simple and leave that part
> as-is
> for now.
Thanks Eric, I was reflecting on that too. I agree that your patch resolves
a problem without introducing a new one.
Reviewed-by: Simon Horman <simon.hor...@netronome.com>
