On Wed, May 22, 2019 at 1:47 PM Kees Cook keescook@chromium.org wrote:
On Wed, May 22, 2019 at 05:35:27PM +0100, Catalin Marinas wrote:
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.
Agreed! And I think the series meets these requirements, yes?
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.
Ah! Okay, here's the use-case I wasn't thinking of: the concern is TBI conflicting with MTE. And anything that starts using TBI suddenly can't run in the future because it's being interpreted as MTE bits? (Is that the ABI concern? I feel like we got into the weeds about ioctl()s and one-off bugs...)
So there needs to be some way to let the kernel know which of three things it should be doing: 1- leaving userspace addresses as-is (present) 2- wiping the top bits before using (this series) 3- wiping the top bits for most things, but retaining them for MTE as needed (the future)
I expect MTE to be the "default" in the future. Once a system's libc has grown support for it, everything will be trying to use MTE. TBI will be the special case (but TBI is effectively a prerequisite).
AFAICT, the only difference I see between 2 and 3 will be the tag handling in usercopy (all other places will continue to ignore the top bits). Is that accurate?
Is "1" a per-process state we want to keep? (I assume not, but rather it is available via no TBI/MTE CONFIG or a boot-time option, if at all?)
To choose between "2" and "3", it seems we need a per-process flag to opt into TBI (and out of MTE). For userspace, how would a future binary choose TBI over MTE? If it's a library issue, we can't use an ELF bit, since the choice may be "late" after ELF load (this implies the need for a prctl().) If it's binary-only ("built with HWKASan") then an ELF bit seems sufficient. And without the marking, I'd expect the kernel to enforce MTE when there are high bits.
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.
I've got the same question as Elliot: aren't MTE instructions just NOP to older CPUs? I.e. if the CPU (or kernel) don't support it, it just gets entirely ignored: checking is only needed to satisfy curiosity or behavioral expectations.
MTE instructions are not NOP. Most of them have side effects (changing register values, zeroing memory). This only matters for stack tagging, though. Heap tagging is a runtime decision in the allocator.
If an image needs to run on old hardware, it will have to do heap tagging only.
To me, the conflict seems to be using TBI in the face of expecting MTE to be the default state of the future. (But the internal changes needed for TBI -- this series -- is a prereq for MTE.)
-- Kees Cook