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

[Catalin Marinas <catalin.marinas@arm.com>]

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.

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?

-- 
Catalin