Re: [PATCH hotfix 6.12 v2 4/8] mm: resolve faulty mmap_region() error path behaviour

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Oct 29, 2024 at 03:04:41PM +0000, Catalin Marinas wrote:
> Hi Lorenzo,
>
> Thanks for trying to fix this.
>
> On Mon, Oct 28, 2024 at 10:14:50PM +0000, Lorenzo Stoakes wrote:
> > From 247003cd2a4b5f4fc2dac97f5ef7e473a47f4324 Mon Sep 17 00:00:00 2001
> > From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > Date: Mon, 28 Oct 2024 22:05:44 +0000
> > Subject: [PATCH] mm: perform MTE check within arm64 hook entirely
> >
> > It doesn't make sense to have shmem explicitly check this one arch-specific
> > case, it is arch-specific, so the arch should handle it. We know shmem is a
> > case in which we want to permit MTE, so simply check for this directly.
> >
> > This also fixes the issue with checking arch_validate_flags() early, which
> > would otherwise break mmap_region().
> >
> > In order to implement this we must pass a file pointer, and additionally
> > update the sparc code to accept this parameter too.
> >
> > We'd ideally like to have eliminated the arch_calc_vm_flag_bits() case, but
> > we risk inadvertently changing behaviour as we do not have mmap() flags
> > available at the point of the arch_validate_flags() check and a MAP_ANON |
> > MAP_HUGETLB case would be accepted for MTE currently, but a MAP_SHARED |
> > MAP_HUGETLB would not.
> >
> > This is likely an oversight but we want to try to keep behaviour identical
> > to before in this patch.
>
> MAP_HUGETLB support for MTE is only in -next currently, so there
> wouldn't be any ABI change if we also allowed MAP_SHARED | MAP_HUGETLB.
> In 6.12, MAP_HUGETLB is not allowed to have PROT_MTE.
>
> > So continue to check VM_MTE_ALLOWED which arch_calc_vm_flag_bits() sets if
> > MAP_ANON.
> [...]
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 4ba1d00fabda..e87f5d6799a7 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -2733,9 +2733,6 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
> >  	if (ret)
> >  		return ret;
> >
> > -	/* arm64 - allow memory tagging on RAM-based files */
> > -	vm_flags_set(vma, VM_MTE_ALLOWED);
>
> This breaks arm64 KVM if the VMM uses shared mappings for the memory
> slots (which is possible). We have kvm_vma_mte_allowed() that checks for
> the VM_MTE_ALLOWED flag as the VMM may not use PROT_MTE/VM_MTE directly.

Ugh yup missed that thanks.

>
> I need to read this thread properly but why not pass the file argument
> to arch_calc_vm_flag_bits() and set VM_MTE_ALLOWED in there?

Can't really do that as it is entangled in a bunch of other stuff,
e.g. calc_vm_prot_bits() would have to pass file and that's used in a bunch
of places including arch code and... etc. etc.

And definitely no good for a hotfix that has to be backported.

I suggest instead we instead don't drop the yucky shmem thing, which will
set VM_MTE_ALLOWED for shmem, with arch_calc_vm_flag_bits() still setting
it for MAP_ANON, but the other changes will mean the arch_validate_flags()
will be fixed too.

So this just means not dropping the mm/shmem.c bit basically and everything
should 'just work'?

>
> --
> Catalin

But we definitely need to find a better way post-hotfix to sort all this
out I'm sure you agree :)