On Fri, May 17, 2019 at 7:49 AM Catalin Marinas catalin.marinas@arm.com wrote:
Hi Andrey,
On Mon, May 06, 2019 at 06:30:46PM +0200, Andrey Konovalov wrote:
One of the alternative approaches to untagging that was considered is to completely strip the pointer tag as the pointer enters the kernel with some kind of a syscall wrapper, but that won't work with the countless number of different ioctl calls. With this approach we would need a custom wrapper for each ioctl variation, which doesn't seem practical.
The more I look at this problem, the less convinced I am that we can solve it in a way that results in a stable ABI covering ioctls(). While for the Android kernel codebase it could be simpler as you don't upgrade the kernel version every 2.5 months, for the mainline kernel this doesn't scale. Any run-time checks are relatively limited in terms of drivers covered. Better static checking would be nice as a long term solution but we didn't get anywhere with the discussion last year.
IMO (RFC for now), I see two ways forward:
Make this a user space problem and do not allow tagged pointers into the syscall ABI. A libc wrapper would have to convert structures, parameters before passing them into the kernel. Note that we can still support the hardware MTE in the kernel by enabling tagged memory ranges, saving/restoring tags etc. but not allowing tagged addresses at the syscall boundary.
Similar shim to the above libc wrapper but inside the kernel (arch/arm64 only; most pointer arguments could be covered with an __SC_CAST similar to the s390 one). There are two differences from what we've discussed in the past:
a) this is an opt-in by the user which would have to explicitly call prctl(). If it returns -ENOTSUPP etc., the user won't be allowed to pass tagged pointers to the kernel. This would probably be the responsibility of the C lib to make sure it doesn't tag heap allocations. If the user did not opt-in, the syscalls are routed through the normal path (no untagging address shim).
b) ioctl() and other blacklisted syscalls (prctl) will not accept tagged pointers (to be documented in Vicenzo's ABI patches).
It doesn't solve the problems we are trying to address but 2.a saves us from blindly relaxing the ABI without knowing how to easily assess new code being merged (over 500K lines between kernel versions). Existing applications (who don't opt-in) won't inadvertently start using the new ABI which could risk becoming de-facto ABI that we need to support on the long run.
Option 1 wouldn't solve the ioctl() problem either and while it makes things simpler for the kernel, I am aware that it's slightly more complicated in user space (but I really don't mind if you prefer option 1 ;)).
The tagged pointers (whether hwasan or MTE) should ideally be a transparent feature for the application writer but I don't think we can solve it entirely and make it seamless for the multitude of ioctls(). I'd say you only opt in to such feature if you know what you are doing and the user code takes care of specific cases like ioctl(), hence the prctl() proposal even for the hwasan.
Comments welcomed.
Any userspace shim approach is problematic for Android because of the apps that use raw system calls. AFAIK, all apps written in Go are in that camp - I'm not sure how common they are, but getting them all recompiled is probably not realistic.
The way I see it, a patch that breaks handling of tagged pointers is not that different from, say, a patch that adds a wild pointer dereference. Both are bugs; the difference is that (a) the former breaks a relatively uncommon target and (b) it's arguably an easier mistake to make. If MTE adoption goes well, (a) will not be the case for long.
This is a bit of a chicken-and-egg problem. In a world where memory allocators on one or several popular platforms generate pointers with non-zero tags, any such breakage will be caught in testing. Unfortunately to reach that state we need the kernel to start accepting tagged pointers first, and then hold on for a couple of years until userspace catches up.
Perhaps we can start by whitelisting ioctls by driver?