Re: [PATCH 5/6] xfrm_user: ensure user supplied esn replay window is valid

On Thu, Sep 20, 2012 at 9:05 AM, Steffen Klassert
[off-list ref] wrote:

On Thu, Sep 20, 2012 at 08:12:11AM +0200, Mathias Krause wrote:

quoted

On Thu, Sep 20, 2012 at 12:38 AM, Ben Hutchings
[off-list ref] wrote:

quoted

On Wed, 2012-09-19 at 23:33 +0200, Mathias Krause wrote:

quoted

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.

Okay. If no-one objects, I'll at add a upper limit check for 4096
packets to verify_replay().

quoted

[...]
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.

The type of the expression calculated in xfrm_replay_state_esn_len()
is size_t; the functions the value get passed onto (k*alloc, kmemdup,
memcpy, memcmp) expect a size_t argument; expressions where the value
is evaluated to calculate sizes (e.g. in xfrm_sa_len) operate on
size_t types. So size_t feels just natural.

Also it should not return anything else than the other xfrm length
calculation functions.

So the other functions should have a return type of size_t, too?

Anyway, such a cleanup should go into a separate patch as the other
functions are not vulnerable to an overflow like it could happen in
xfrm_replay_state_esn_len().

Once we limited bmp_len, xfrm_replay_state_esn_len() should return
always a positive value.

True. So int it'll be then again for xfrm_replay_state_esn_len() in v3
of the patch.


Thanks,
Mathias