On 2/6/19 1:48 PM, Julien Gomes wrote: > > > On 2/6/19 1:39 PM, Neil Horman wrote: >> On Wed, Feb 06, 2019 at 01:26:55PM -0800, Julien Gomes wrote: >>> >>> >>> On 2/6/19 1:07 PM, Marcelo Ricardo Leitner wrote: >>>> On Wed, Feb 06, 2019 at 12:48:38PM -0800, Julien Gomes wrote: >>>>> >>>>> >>>>> On 2/6/19 12:37 PM, Marcelo Ricardo Leitner wrote: >>>>>> On Wed, Feb 06, 2019 at 12:14:30PM -0800, Julien Gomes wrote: >>>>>>> Make sctp_setsockopt_events() able to accept sctp_event_subscribe >>>>>>> structures longer than the current definitions. >>>>>>> >>>>>>> This should prevent unjustified setsockopt() failures due to struct >>>>>>> sctp_event_subscribe extensions (as in 4.11 and 4.12) when using >>>>>>> binaries that should be compatible, but were built with later kernel >>>>>>> uapi headers. >>>>>> >>>>>> Not sure if we support backwards compatibility like this? >>>>>> >>>>>> My issue with this change is that by doing this, application will have >>>>>> no clue if the new bits were ignored or not and it may think that an >>>>>> event is enabled while it is not. >>>>>> >>>>>> A workaround would be to do a getsockopt and check the size that was >>>>>> returned. But then, it might as well use the right struct here in the >>>>>> first place. >>>>>> >>>>>> I'm seeing current implementation as an implicitly versioned argument: >>>>>> it will always accept setsockopt calls with an old struct (v4.11 or >>>>>> v4.12), but if the user tries to use v3 on a v1-only system, it will >>>>>> be rejected. Pretty much like using a newer setsockopt on an old >>>>>> system. >>>>> >>>>> With the current implementation, given sources that say are supposed to >>>>> run on a 4.9 kernel (no use of any newer field added in 4.11 or 4.12), >>>>> we can't rebuild the exact same sources on a 4.19 kernel and still run >>>>> them on 4.9 without messing with structures re-definition. >>>> >>>> Maybe what we want(ed) here then is explicit versioning, to have the 3 >>>> definitions available. Then the application is able to use, say struct >>>> sctp_event_subscribe, and be happy with it, while there is struct >>>> sctp_event_subscribe_v2 and struct sctp_event_subscribe_v3 there too. >>>> >>>> But it's too late for that now because that would break applications >>>> already using the new fields in sctp_event_subscribe. >>> >>> Right. >>> >>>> >>>>> >>>>> I understand your point, but this still looks like a sort of uapi >>>>> breakage to me. >>>> >>>> Not disagreeing. I really just don't know how supported that is. >>>> Willing to know so I can pay more attention to this on future changes. >>>> >>>> Btw, is this the only occurrence? >>> >>> Can't really say, this is one I witnessed, I haven't really looked for >>> others. >>> >>>> >>>>> >>>>> >>>>> I also had another way to work-around this in mind, by copying optlen >>>>> bytes and checking that any additional field (not included in the >>>>> "current" kernel structure definition) is not set, returning EINVAL in >>>>> such case to keep a similar to current behavior. >>>>> The issue with this is that I didn't find a suitable (ie not totally >>>>> arbitrary such as "twice the existing structure size") upper limit to >>>>> optlen. >>>> >>>> Seems interesting. Why would it need that upper limit to optlen? >>>> >>>> Say struct v1 had 4 bytes, v3 now had 12. The user supplies 12 bytes >>>> to the kernel that only knows about 4 bytes. It can check that (12-4) >>>> bytes in the end, make sure no bit is on and use only the first 4. >>>> >>>> The fact that it was 12 or 200 shouldn't matter, should it? As long as >>>> the (200-4) bytes are 0'ed, only the first 4 will be used and it >>>> should be ok, otherwise EINVAL. No need to know how big the current >>>> current actually is because it wouldn't be validating that here: just >>>> that it can safely use the first 4 bytes. >>> >>> The upper limit concern is more regarding the call to copy_from_user >>> with an unrestricted/unchecked value. >> Copy_from_user should be safe to copy an arbitrary amount, the only restriction >> is that optlen can't exceed the size of the buffer receiving the data in the >> kernel. From that standpoint your patch is safe. However, that exposes the >> problem of checking any tail data on the userspace buffer. That is to say, if >> you want to ensure that any extra data that gets sent from userspace isn't >> 'set', you would have to copy that extra data in consumable chunks and check >> them individaully, and that screams DOS to me (i.e. imagine a user passing in a >> 4GB buffer, and having to wait for the kernel to copy each X sized chunk, >> looking for non-zero values). > > There probably is a decent compromise to find between "not accepting a > single additional byte" and accepting several GB. > For example how likely is it that the growth of this structure make it > go over a page? I would hope not at all. > > By choosing a large but decent high limit, I think we can find a > future-compatible compromise that doesn't rely on a preliminary > getsockopt() just for structure trucation decision... And I was just reminded about huge pages. But still, my point of finding a compromise still stands. > >> >>> I am not sure of how much of a risk/how exploitable this could be, >>> that's why I cautiously wanted to limit it in the first place just in case. >>> >>>> >>>>> >>>>>> >>>>>>> >>>>>>> Signed-off-by: Julien Gomes <julien@xxxxxxxxxx> >>>>>>> --- >>>>>>> net/sctp/socket.c | 2 +- >>>>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>>>> >>>>>>> diff --git a/net/sctp/socket.c b/net/sctp/socket.c >>>>>>> index 9644bdc8e85c..f9717e2789da 100644 >>>>>>> --- a/net/sctp/socket.c >>>>>>> +++ b/net/sctp/socket.c >>>>>>> @@ -2311,7 +2311,7 @@ static int sctp_setsockopt_events(struct sock *sk, char __user *optval, >>>>>>> int i; >>>>>>> >>>>>>> if (optlen > sizeof(struct sctp_event_subscribe)) >>>>>>> - return -EINVAL; >>>>>>> + optlen = sizeof(struct sctp_event_subscribe); >>>>>>> >>>>>>> if (copy_from_user(&subscribe, optval, optlen)) >>>>>>> return -EFAULT; >>>>>>> -- >>>>>>> 2.20.1 >>>>>>> >>>>> >>> >>> -- >>> Julien Gomes >>> >
