Re: [PATCH RFC v2 18/27] arm64: mte: Reserve tag block for the zero page

[David Hildenbrand]

On 29.11.23 12:30, Alexandru Elisei wrote:
> On Tue, Nov 28, 2023 at 06:06:54PM +0100, David Hildenbrand wrote:
> > On 19.11.23 17:57, Alexandru Elisei wrote:
> > > On arm64, the zero page receives special treatment by having the tagged
> > > flag set on MTE initialization, not when the page is mapped in a process
> > > address space. Reserve the corresponding tag block when tag storage
> > > management is being activated.
> >
> > Out of curiosity: why does the shared zeropage require tagged storage? What
> > about the huge zeropage?
>
> There are two different tags that are used for tag checking: the logical
> tag, the tag embedded in bits 59:56 of an address, and the physical tag
> corresponding to the address. This tag is stored in a separate memory
> location, called tag storage. When an access is performed, hardware
> compares the logical tag (from the address) with the physical tag (from the
> tag storage). If they match, the access is permitted.

Ack, matches my understanding.

> The physical tag is set with special instructions.
>
> Userspace pointers have bits 59:56 zero. If the pointer is in a VMA with
> MTE enabled, then for userspace to be able to access this address, the
> physical tag must also be 0b0000.
>
> To make it easier on userspace, when a page is first mapped as tagged, its
> tags are cleared by the kernel; this way, userspace can access the address
> immediately, without clearing the physical tags beforehand. Another reason
> for clearing the physical tags when a page is mapped as tagged would be to
> avoid leaking uninitialized tags to userspace.

Make sense. Zero it just like we zero page content.

> The zero page is special, because the physical tags are not zeroed every
> time the page is mapped in a process; instead, the zero page is marked as
> tagged (by setting a page flag) and the physical tags are zeroed only once,
> when MTE is enabled at boot.

Makes sense.

> All of this means that when tag storage is enabled, which happens after MTE
> is enabled, the tag storage corresponding to the zero page is already in
> use and must be rezerved, and it can never be used for data allocations.
>
> I hope all of the above makes sense. I can also put it in the commit
> message :)

Yes, makes sense!

> As for the zero huge page, the MTE code in the kernel treats it like a
> regular page, and it zeroes the tags when it is mapped as tagged in a
> process. I agree that this might not be the best solution from a
> performance perspective, but it has worked so far.

What if user space were to change the tag of that shared resource?

Having a tag != 0 doesn't make sense for such a shared resource, so I 
suspect modifying the tag is like a write event: trigger write-fault -> COW.

> With tag storage management enabled, set_pte_at()->mte_sync_tags() will
> discover that the huge zero page doesn't have tag storage reserved, the
> table entry will be mapped as invalid to use the page fault-on-access
> mechanism that I introduce later in the series [1] to reserve tag storage,

I assume (without looking at the code) that you took proper care of 
possible races.

Thanks for goind into detail!


--
Cheers,

David / dhildenb