arm64: pageattr: Explicitly bail out when changing permissions for vmalloc_huge mappings

arm64 uses apply_to_page_range to change permissions for kernel vmalloc mappings,
which does not support changing permissions for block mappings. This function
will change permissions until it encounters a block mapping, and will bail
out with a warning. Since there are no reports of this triggering, it
implies that there are currently no cases of code doing a vmalloc_huge()
followed by partial permission change. But this is a footgun waiting to
go off, so let's detect it early and avoid the possibility of permissions
in an intermediate state. So,  explicitly disallow changing permissions
for VM_ALLOW_HUGE_VMAP mappings.

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20250403052844.61818-1-dev.jain@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: Dev Jain

arch/arm64/mm/pageattr.c

@@ -96,16 +96,16 @@ static int change_memory_common(unsigned long addr, int numpages,
 	 * we are operating on does not result in such splitting.
 	 *
 	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
-	 * Those are guaranteed to consist entirely of page mappings, and
-	 * splitting is never needed.
+	 * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
+	 * mappings are updated and splitting is never needed.
 	 *
 	 * So check whether the [addr, addr + size) interval is entirely
 	 * covered by precisely one VM area that has the VM_ALLOC flag set.
 	 */
 	area = find_vm_area((void *)addr);
 	if (!area ||
 	    end > (unsigned long)kasan_reset_tag(area->addr) + area->size ||
-	    !(area->flags & VM_ALLOC))
+	    ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
 		return -EINVAL;
 
 	if (!numpages)