Re: [RFC PATCH 1/4] mm/mprotect: add MM_CP_PERM_UPGRADE

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] mm/mprotect: MM_CP_PERM_UPGRADE set when change_protection() revokes write
- [Medium] mm/mprotect: vma_flags_is_perm_upgrade() ignores arch-specific prot flags
--

commit f3d5a380cac712469a433d985427420721eea496
Author: Carlos López <clopez@suse.de>

mm/mprotect: add MM_CP_PERM_UPGRADE

This commit introduces the MM_CP_PERM_UPGRADE flag for change_protection()
when mprotect() updates VMA permissions to a superset of the previous
permissions without changing the protection key, allowing MMU notifier
consumers to avoid invalidating PTEs.

> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 754333698c1ea..96e32e0159166 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -756,6 +756,17 @@ static const struct mm_walk_ops prot_none_walk_ops = {
>  	.walk_lock		= PGWALK_WRLOCK,
>  };
>  
> +static bool vma_flags_is_perm_upgrade(vma_flags_t old_vma_flags,
> +				       vma_flags_t new_vma_flags)
> +{
> +	vm_flags_t old = vma_flags_to_legacy(old_vma_flags);
> +	vm_flags_t new = vma_flags_to_legacy(new_vma_flags);
> +	vm_flags_t rwx = VM_READ | VM_WRITE | VM_EXEC;
> +
> +	return !(old & rwx & ~new) &&
> +	       (old & ARCH_VM_PKEY_FLAGS) == (new & ARCH_VM_PKEY_FLAGS);
> +}

[Severity: Medium]
Does vma_flags_is_perm_upgrade() also need to check architecture-specific
protection flags that mprotect() can modify?

In do_mprotect_pkey(), newflags is computed via calc_vm_prot_bits() (which
calls arch_calc_vm_prot_bits()) and masks off VM_FLAGS_CLEAR (which includes
VM_ARCH_CLEAR):

mm/mprotect.c:do_mprotect_pkey() {
    ...
    mask_off_old_flags = VM_ACCESS_FLAGS | VM_FLAGS_CLEAR;

    new_vma_pkey = arch_override_mprotect_pkey(vma, prot, pkey);
    newflags = calc_vm_prot_bits(prot, new_vma_pkey);
    newflags |= (vma->vm_flags & ~mask_off_old_flags);
    ...
}

Several architectures expose PROT_* flags in mprotect() that set or clear
VMA flags affecting hardware PTE protection or attribute bits in
vm_get_page_prot():

- On arm64, arch_calc_vm_prot_bits() maps PROT_BTI to VM_ARM64_BTI (cleared
  via VM_ARCH_CLEAR when omitted) and PROT_MTE to VM_MTE. In
  arch/arm64/mm/mmap.c:vm_get_page_prot(), VM_ARM64_BTI sets PTE_GP (which
  restricts execution by faulting on indirect branches to non-BTI landing
  pads) and VM_MTE sets PTE_ATTRINDX(MT_NORMAL_TAGGED) (which enables MTE
  tag check faults).
- On sparc64, PROT_ADI sets VM_SPARC_ADI (cleared via VM_ARCH_CLEAR when
  omitted), which sets _PAGE_MCD_4V in
  arch/sparc/mm/init_64.c:vm_get_page_prot() to enable hardware ADI tag
  checking traps.
- On powerpc64, PROT_SAO sets VM_SAO, which sets _PAGE_SAO in
  arch/powerpc/mm/book3s64/pgtable.c:vm_get_page_prot() to alter hardware
  memory ordering attributes.

Because vma_flags_is_perm_upgrade() only inspects VM_READ | VM_WRITE |
VM_EXEC and ARCH_VM_PKEY_FLAGS, if userspace calls mprotect() to enable
PROT_BTI, PROT_MTE, PROT_ADI, or PROT_SAO while keeping the read/write/exec
bits identical (or a superset) and the pkey unchanged, won't this set
MM_CP_PERM_UPGRADE and allow MMU notifier consumers to skip secondary MMU
invalidation when access restrictions were tightened or PTE attributes
changed?

> +
>  int
>  mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
>  	       struct vm_area_struct *vma, struct vm_area_struct **pprev,
> @@ -836,6 +847,8 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
>  	vma_flags_reset_once(vma, &new_vma_flags);
>  	if (vma_wants_manual_pte_write_upgrade(vma))
>  		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
> +	if (vma_flags_is_perm_upgrade(old_vma_flags, new_vma_flags))
> +		mm_cp_flags |= MM_CP_PERM_UPGRADE;
>  	vma_set_page_prot(vma);
>  
>  	change_protection(tlb, vma, start, end, mm_cp_flags);

[Severity: High]
Can setting MM_CP_PERM_UPGRADE when the VMA already had VM_WRITE (or on a
hugetlb VMA) violate the invariant that no access is revoked and the PFN
stays the same?

Follow-up commits ("mm/mmu_notifier: add MMU_NOTIFIER_RANGE_PERM_UPGRADE",
"KVM: plumb MMU_NOTIFIER_RANGE_PERM_UPGRADE into kvm_gfn_range", and
"KVM: x86/mmu: don't zap the secondary MMU on permission upgrade") wire
MM_CP_PERM_UPGRADE to MMU_NOTIFIER_RANGE_PERM_UPGRADE so that
kvm_unmap_gfn_range() skips zapping secondary MMU SPTEs.

However, when a VMA already has VM_WRITE and mprotect() adds permissions
(for example, upgrading PROT_READ | PROT_WRITE to
PROT_READ | PROT_WRITE | PROT_EXEC, or PROT_WRITE to
PROT_READ | PROT_WRITE), vma_set_page_prot() computes vma->vm_page_prot
(newprot) without write permission for all MAP_PRIVATE mappings (COW) and
for MAP_SHARED mappings where vma_wants_writenotify() is true.

When change_protection() applies this read-only newprot across the VMA,
there are several paths where write permission is stripped from existing
entries or entries are unmapped in the primary MMU while MM_CP_PERM_UPGRADE
is set:

1. In mprotect_fixup(), vma_wants_manual_pte_write_upgrade(vma) is called
   after vma_flags_reset_once() but before vma_set_page_prot(vma). For
   MAP_SHARED VMAs without vm_ops->page_mkwrite that use writenotify for
   VM_UFFD_WP or soft-dirty tracking (such as shmem),
   vma_wants_writenotify(vma, vma->vm_page_prot) compares the old read-only
   vma->vm_page_prot against vma_pgprot_modify(vm_page_prot, vma->flags)
   (which still has VMA_SHARED_BIT | VMA_WRITE_BIT) and returns false. As a
   result, MM_CP_TRY_CHANGE_WRITABLE is not set, vma_set_page_prot()
   computes a read-only vma->vm_page_prot, and change_present_ptes() /
   change_huge_pmd() strip write permission from all writable PTEs and PMDs
   in the VMA.

2. In mm/hugetlb.c:hugetlb_change_protection(), MM_CP_TRY_CHANGE_WRITABLE
   is not handled, so huge_pte_modify(old_pte, newprot) unconditionally
   strips write permission from already-writable COW huge PTEs in
   MAP_PRIVATE hugetlb VMAs; in addition, huge_pmd_unshare() unmaps shared
   PMD tables via pud_clear().

3. In mm/huge_memory.c:change_huge_pud(), MM_CP_TRY_CHANGE_WRITABLE is not
   handled, so pud_modify(oldpud, newprot) strips write permission from
   writable huge PUDs in writenotify DAX mappings.

4. In mm/mprotect.c:change_softleaf_pte(),
   mm/huge_memory.c:change_non_present_huge_pmd(), and
   mm/hugetlb.c:hugetlb_change_protection(), writable migration entries
   (softleaf_is_migration_write()) and writable device-private entries
   (softleaf_is_device_private_write()) are unconditionally downgraded to
   read-only entries via make_readable_exclusive_migration_entry(),
   make_readable_migration_entry(), or make_readable_device_private_entry().

5. In change_present_ptes() and change_huge_pmd(), pte_modify() /
   pmd_modify() strip write permission and do not restore it under
   MM_CP_TRY_CHANGE_WRITABLE when !folio in a private PFN/mixed mapping or
   when a shared writenotify PTE was cleaned (!pte_dirty(ptent)).

Wouldn't skipping secondary MMU invalidation in these cases leave secondary
MMUs (such as KVM SPTEs) with stale writable mappings to pages that
change_protection() has write-protected or unmapped in the primary MMU?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261029143336.390231-3-clopez@suse.de?part=1