On Wed, May 22, 2019 at 9:35 AM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > On Wed, May 22, 2019 at 08:30:21AM -0700, enh wrote: > > On Wed, May 22, 2019 at 3:11 AM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > > On Tue, May 21, 2019 at 05:04:39PM -0700, Kees Cook wrote: > > > > I just want to make sure I fully understand your concern about this > > > > being an ABI break, and I work best with examples. The closest situation > > > > I can see would be: > > > > > > > > - some program has no idea about MTE > > > > > > Apart from some libraries like libc (and maybe those that handle > > > specific device ioctls), I think most programs should have no idea about > > > MTE. I wouldn't expect programmers to have to change their app just > > > because we have a new feature that colours heap allocations. > > > > obviously i'm biased as a libc maintainer, but... > > > > i don't think it helps to move this to libc --- now you just have an > > extra dependency where to have a guaranteed working system you need to > > update your kernel and libc together. (or at least update your libc to > > understand new ioctls etc _before_ you can update your kernel.) > > That's not what I meant (or I misunderstood you). If we have a relaxed > ABI in the kernel and a libc that returns tagged pointers on malloc() I > wouldn't expect the programmer to do anything different in the > application code like explicit untagging. Basically the program would > continue to run unmodified irrespective of whether you use an old libc > without tagged pointers or a new one which tags heap allocations. > > What I do expect is that the libc checks for the presence of the relaxed > ABI, currently proposed as an AT_FLAGS bit (for MTE we'd have a > HWCAP_MTE), and only tag the malloc() pointers if the kernel supports > the relaxed ABI. As you said, you shouldn't expect that the C library > and kernel are upgraded together, so they should be able to work in any > new/old version combination. yes, that part makes sense. i do think we'd use the AT_FLAGS bit, for exactly this. i was questioning the argument about the ioctl issues, and saying that from my perspective, untagging bugs are not really any different than any other kind of kernel bug. > > > > The trouble I see with this is that it is largely theoretical and > > > > requires part of userspace to collude to start using a new CPU feature > > > > that tickles a bug in the kernel. As I understand the golden rule, > > > > this is a bug in the kernel (a missed ioctl() or such) to be fixed, > > > > not a global breaking of some userspace behavior. > > > > > > Yes, we should follow the rule that it's a kernel bug but it doesn't > > > help the user that a newly installed kernel causes user space to no > > > longer reach a prompt. Hence the proposal of an opt-in via personality > > > (for MTE we would need an explicit opt-in by the user anyway since the > > > top byte is no longer ignored but checked against the allocation tag). > > > > but realistically would this actually get used in this way? or would > > any given system either be MTE or non-MTE. in which case a kernel > > configuration option would seem to make more sense. (because either > > way, the hypothetical user basically needs to recompile the kernel to > > get back on their feet. or all of userspace.) > > The two hard requirements I have for supporting any new hardware feature > in Linux are (1) a single kernel image binary continues to run on old > hardware while making use of the new feature if available and (2) old > user space continues to run on new hardware while new user space can > take advantage of the new feature. > > The distro user space usually has a hard requirement that it continues > to run on (certain) old hardware. We can't enforce this in the kernel > but we offer the option to user space developers of checking feature > availability through HWCAP bits. > > The Android story may be different as you have more control about which > kernel configurations are deployed on specific SoCs. I'm looking more > from a Linux distro angle where you just get an off-the-shelf OS image > and install it on your hardware, either taking advantage of new features > or just not using them if the software was not updated. Or, if updated > software is installed on old hardware, it would just run. > > For MTE, we just can't enable it by default since there are applications > who use the top byte of a pointer and expect it to be ignored rather > than failing with a mismatched tag. Just think of a hwasan compiled > binary where TBI is expected to work and you try to run it with MTE > turned on. > > I would also expect the C library or dynamic loader to check for the > presence of a HWCAP_MTE bit before starting to tag memory allocations, > otherwise it would get SIGILL on the first MTE instruction it tries to > execute. (a bit off-topic, but i thought the MTE instructions were encoded in the no-op space, to avoid this?) > > i'm not sure i see this new way for a kernel update to break my system > > and need to be fixed forward/rolled back as any different from any of > > the existing ways in which this can happen :-) as an end-user i have > > to rely on whoever's sending me software updates to test adequately > > enough that they find the problems. as an end user, there isn't any > > difference between "my phone rebooted when i tried to take a photo > > because of a kernel/driver leak", say, and "my phone rebooted when i > > tried to take a photo because of missing untagging of a pointer passed > > via ioctl". > > > > i suspect you and i have very different people in mind when we say "user" :-) > > Indeed, I think we have different users in mind. I didn't mean the end > user who doesn't really care which C library version it's running on > their phone but rather advanced users (not necessarily kernel > developers) that prefer to build their own kernels with every release. > We could extend this to kernel developers who don't have time to track > down why a new kernel triggers lots of SIGSEGVs during boot. i still don't see how this isn't just a regular testing/CI issue, the same as any other kind of kernel bug. it's already the case that i can get a bad kernel... > -- > Catalin
