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

[Alexandru Elisei]

Hi,

On Wed, Nov 29, 2023 at 02:13:50PM +0100, David Hildenbrand wrote:
> 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.

Yes, modifying the tag is a write event.

> 
> > 
> > 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!

No problem.

Alex

> 
> 
> -- 
> Cheers,
> 
> David / dhildenb
>