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>

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 39fd1f7ff02aa0370df97b689c1d22d73bc34cc7..04d4a8f676db422d3e0593719e75c36f52a9a641 100644 (file)
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -96,8 +96,8 @@ 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.
@@ -105,7 +105,7 @@ static int change_memory_common(unsigned long addr, int numpages,
        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)