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... > >> 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 >> -- Julien Gomes
