On Mon, Feb 11, 2019 at 10:04:32AM -0500, Neil Horman wrote: > On Sun, Feb 10, 2019 at 10:46:16AM -0200, Marcelo Ricardo Leitner wrote: > > On Sat, Feb 09, 2019 at 03:12:17PM -0800, David Miller wrote: > > > From: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx> > > > Date: Wed, 6 Feb 2019 18:37:54 -0200 > > > > > > > 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? > > > > > > What a complete mess we have here. > > > > > > Use new socket option numbers next time, do not change the size and/or > > > layout of existing socket options. > > > > What about reusing the same socket option, but defining a new struct? > > Say, MYSOCKOPT supports struct mysockopt, struct mysockopt2, struct > > mysockopt3... > > > > That way we have a clear definition of the user's intent. > > > Thats possible, but I think thats pretty equivalaent to what daves saying, in > that he wants us to identify all the sizes of this struct and the git history > and act on them accordingly. Having internal versions of the struct seems like > a fine way to get there, but I think we need to consider how we got to this > situations before we go down the implementation path. I was more referring to future stuff, but yes. I find it a bit easier to handle than having to switch the sockopt too and so far I couldn't find drawbacks to it. That is, when using a new sockopt, we could accept a buffer larger than the needed, but I'm not considering that as a valid point anymore. Putting this compatibility aside for a moment, that pretty much means the user doesn't know what it wants and so we also don't. > > > > > > > This whole thread, if you read it, is basically "if we compatability > > > this way, that breaks, and if we do compatability this other way oh > > > shit this other thing doesn't work." > > > > > > I think we really need to specifically check for the difference sizes > > > that existed one by one, clear out the part not given by the user, and > > > backport this as far back as possible in a way that in the older kernels > > > we see if the user is actually trying to use the new features and if so > > > error out. > > > > I'm afraid clearing out may not be enough, though seems it's the best > > we can do so far. If the struct is allocated but not fully initialized > > via a memset, but by setting its fields one by one, the remaining new > > fields will be left uninitinialized. > > > > I'm not sure this even makes sense. Currently (as I understood it), the issue > we are facing is the one in which an application is built against a newer kernel > and run on an older one, the implication there being that the application will > pass in a buffer that is larger than what the kernel expects. In that > situation, clearing isn't needed, all thats needed (I think), is a memcmp of the > space between the sizeof(kernel struct version), and sizeof(userspace struct > version) to see if any bits are non-zero. If they are, we error out, otherwise, > we ignore the space and move forward as though that overage doesn't exist. That's exactly what I tried to mean. :-) > > Mind you, I'm not (yet) advocating for that approach, just trying to clarify > whats needed. Ok. > > > > > > Which, btw, is terrible behavior. Newly compiled apps should work on > > > older kernels if they don't try to use the new features, and if they > > > > One use case here is: a given distro is using kernel X and app Foo is > > built against it. Then upgrades to X+1, Foo is patched to fix an issue > > and is rebuilt against X+1. The user upgrades Foo package but for > > whatever reason, doesn't upgrade kernel or reboot the system. Here, > > Foo doesn't work anymore until the new kernel is also running. > > > Yes, thats the use case that we're trying to address. > > > > can the ones that want to try to use the new features should be able > > > to fall back when that feature isn't available in a non-ambiguous > > > and precisely defined way. > > > > > > The fact that the use of the new feature is hidden in the new > > > structure elements is really rotten. > > > > > > This patch, at best, needs some work and definitely a longer and more > > > detailed commit message. > > > FWIW, before we decide on a course of action, I think I need to point out that, > over the last 10 years, we've extended this structure 6 times, in the following > commits: > 0f3fffd8ab1db > 7e8616d8e7731 > e1cdd553d482c > 35ea82d611da5 > c95129d127c6d > b444153fb5a64 > > The first two I believe were modifications during a period when sctp was > actually getting integrated to the kernel, but the last 4 were definately done > during more recent development periods and wen't in without any commentary about > the impact to UAPI compatibility. The check for optlen > sizeof(struct > sctp_event_subscribe) was made back in 2008, and while not spelled out, seems > pretty clearly directed at enforcing compatibility with older appliations, not > compatibility with newer applications running on older kernels. > > I really worry about situations in which we need to support applications > expecting features that the running kernel doesn't have. In this particular > situation it seems like a fixable thing, but I could envision situations in > which we just can't do it, and I don't want to set that expectation when we > can't consistently meet it. > > So, if the consensus is that we need to support applications built on newer > kernels, but run on older kernels (and I'd like to get verbal consensus on Yes from my side. > that), then we need to identify a method to fix this. I'm still hesitant to > do anything that involves us accepting any size buffer over the kernel expected > size, as that puts us in a position to have to read large amounts of user data > (i.e. possible DOS), and just picking an arbitrary large number to limit the > buffer size seems wrong. What if, on receipt of a structure from a newer kernel > (implying a size larger than what the kernel expects), we clamp optlen to the > kernel size, and put_user it back to the application? i.e. we don't check any We can't do that on setsockopt calls, as optlen is R/O there. Returning > 0 is not specified on setsockopt(2). > data above and beyond what the the kernel knows about, but we use the optlen as > an indicator to user space that not all the data was processed? That allows the > kernel to ignore the overage safely, and while its not in the socket api > extension RFC, its not violating anything, and is something we can document in > the sctp(7) man page as a linux only behavior. > > Thoughts? > Neil I also need to dig deeper on this, but in general what if we draw a line based on the current implementation: - Current struct is X bytes long - Patch current and older kernels to accept up to X bytes, as long as the trailing bytes are zeroed. Otherwise, EINVAL. X may be a magic number for old kernel, but this way we avoid unbounded buffers and the limit is not random. - On further changes, create a new, explicitly versioned struct. Older kernels will EINVAL if this new struct is used, which is expected. Newer kernels will then have to cope with the different sizes/structs accordingly. Marcelo
