[PATCH v2] arm64: pageattr: Explicitly bail out when changing permissions for vmalloc_huge mappings

[Dev Jain]

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@xxxxxxx>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
v1->v2:
 - Improve changelog, keep mention of page mappings in comment

 arch/arm64/mm/pageattr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 39fd1f7ff02a..04d4a8f676db 100644
--- 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)
-- 
2.30.2