Messages in this thread | | | Date | Thu, 20 Sep 2012 09:05:09 +0200 | From | Steffen Klassert <> | Subject | Re: [PATCH 5/6] xfrm_user: ensure user supplied esn replay window is valid |
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On Thu, Sep 20, 2012 at 08:12:11AM +0200, Mathias Krause wrote: > On Thu, Sep 20, 2012 at 12:38 AM, Ben Hutchings > <bhutchings@solarflare.com> wrote: > > On Wed, 2012-09-19 at 23:33 +0200, Mathias Krause wrote: > > > I'm a little worried that the user-provided > > xfrm_replay_state_esn::bmp_len is not being directly validated anywhere. > > That's what my P.S. in the cover letter tried to hint at -- a missing > upper limit check. But as I wanted to avoid lengthy discussions about > the concrete value and the possible need for some sysctl knob to tune > this even further, I just left this as an exercise for someone else > who is more familiar with the code ;) >
I think we should limit bmp_len to some sane value. RFC 4303 recommends an anti replay window size of 64 packets, so limiting bmp_len to cover 4096 packets should be more that enough. Also we can increase this value later without changing the user API if this is needed.
> > Currently xfrm_replay_state_esn_len() may overflow, and as its return > > type is int it may unexpectedly return a negative value. > > So xfrm_replay_state_esn_len() should return size_t instead as it's > value should always be positive -- it represents a length. Negative > lengths make no sense. It can overflow, still. But it cannot get > negative, at least. Still, the upper limit check would be required to > avoid other user induced nastiness. > > > > > [...] > >> --- a/net/xfrm/xfrm_user.c > >> +++ b/net/xfrm/xfrm_user.c > > [...] > >> @@ -370,14 +378,15 @@ static inline int xfrm_replay_verify_len(struct xfrm_replay_state_esn *replay_es > >> struct nlattr *rp) > >> { > >> struct xfrm_replay_state_esn *up; > >> + size_t ulen; > > > > I would normally expect to see sizes declared as size_t but mixing > > size_t and int in comparisons tends to result in bugs. So I think this > > should to be int, matching the return types of nla_len() and > > xfrm_replay_state_esn_len() (and apparently all lengths in netlink...) > > I disagree. The value of nla_len() is ensured to be in the range of > [sizeof(*up), USHRT_MAX-NLA_HDRLEN], i.e. a positive 16 bit number, > when it passes nlmsg_parse() in xfrm_user_rcv_msg(). This in turn > allows us to assume the int value returned by nla_len() is actually > positive and the compiler can safely make it unsigned for the compare > -- no sign bit, no hassle.
I think xfrm_replay_state_esn_len() should return the same type as nla_len(), no matter what we can assume from the current code base. Also it should not return anything else than the other xfrm length calculation functions.
Once we limited bmp_len, xfrm_replay_state_esn_len() should return always a positive value.
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