Subject: Re: [PATCH v4 11/26] arm64: mte: Add PROT_MTE support to mmap() and mprotect()

From: Catalin Marinas <[email-masked]>
Date: Thu, 28 May 2020 17:34:13 +0100

On Thu, May 28, 2020 at 12:05:09PM +0100, Szabolcs Nagy wrote:
> The 05/28/2020 10:14, Catalin Marinas wrote:
> > On Wed, May 27, 2020 at 11:57:39AM -0700, Peter Collingbourne wrote:
> > > On Fri, May 15, 2020 at 10:16 AM Catalin Marinas
> > > <[email-masked]> wrote:
> > > > To enable tagging on a memory range, the user must explicitly opt in via
> > > > a new PROT_MTE flag passed to mmap() or mprotect(). Since this is a new
> > > > memory type in the AttrIndx field of a pte, simplify the or'ing of these
> > > > bits over the protection_map[] attributes by making MT_NORMAL index 0.
> > > 
> > > Should the userspace stack always be mapped as if with PROT_MTE if the
> > > hardware supports it? Such a change would be invisible to non-MTE
> > > aware userspace since it would already need to opt in to tag checking
> > > via prctl. This would let userspace avoid a complex stack
> > > initialization sequence when running with stack tagging enabled on the
> > > main thread.
> > 
> > I don't think the stack initialisation is that difficult. On program
> > startup (can be the dynamic loader). Something like (untested):
> > 
> > 	register unsigned long stack asm ("sp");
> > 	unsigned long page_sz = sysconf(_SC_PAGESIZE);
> > 
> > 	mprotect((void *)(stack & ~(page_sz - 1)), page_sz,
> > 		 PROT_READ | PROT_WRITE | PROT_MTE | PROT_GROWSDOWN);
> > 
> > (the essential part it PROT_GROWSDOWN so that you don't have to specify
> > a stack lower limit)
> 
> does this work even if the currently mapped stack is more than page_sz?
> determining the mapped main stack area is i think non-trivial to do in
> userspace (requires parsing /proc/self/maps or similar).

Because of PROT_GROWSDOWN, the kernel adjusts the start of the range
down automatically. It is potentially problematic if the top of the
stack is more than a page away and you want the whole stack coloured. I
haven't run a test but my reading of the kernel code is that the stack
vma would be split in this scenario, so the range beyond sp+page_sz
won't have PROT_MTE set.

My assumption is that if you do this during program start, the stack is
smaller than a page. Alternatively, could we use argv or envp to
determine the top of the user stack (the bottom is taken care of by the
kernel)?

> > I'm fine, however, with enabling PROT_MTE on the main stack based on
> > some ELF note.
> 
> note that would likely mean an elf note on the dynamic linker
> (because a dynamic linked executable may not be loaded by the
> kernel and ctors in loaded libs run before the executable entry
> code anyway, so the executable alone cannot be in charge of this
> decision) i.e. one global switch for all dynamic linked binaries.

I guess parsing such note in the kernel is only useful for static
binaries.

> i think a dynamic linker can map a new stack and switch to it
> if it needs to control the properties of the stack at runtime
> (it's wasteful though).

There is already user code to check for HWCAP2_MTE and the prctl(), so
adding an mprotect() doesn't look like a significant overhead.

> and i think there should be a runtime mechanism for the brk area:
> it should be possible to request that future brk expansions are
> mapped as PROT_MTE so an mte aware malloc implementation can use
> brk. i think this is not important in the initial design, but if
> a prctl flag can do it that may be useful to add (may be at a
> later time).

Looking at the kernel code briefly, I think this would work. We do end
up with two vmas for the brk, only the expansion having PROT_MTE, and
I'd to find a way to store the extra flag.

>From a coding perspective, it's easier to just set PROT_MTE by default
on both brk and initial stack ;) (VM_DATA_DEFAULT_FLAGS).

> (and eventually there should be a way to use PROT_MTE on
> writable global data and appropriate code generation that
> takes colors into account when globals are accessed, but
> that requires significant ELF, ld.so and compiler changes,
> that need not be part of the initial mte design).

The .data section needs to be driven by the ELF information. It's also a
file mapping and we don't support PROT_MTE on them even if MAP_PRIVATE.
There are complications like DAX where the file you mmap for CoW may be
hosted on memory that does not support MTE (copied to RAM on write).

Is there a use-case for global data to be tagged?

-- 
Catalin

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