[RFC PATCH 5/9] arm64: mm: Permit permissions changes on huge vmappings

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

Allow permission changes on huge vmappings in cases where no splitting
is needed (i.e., the region is aligned sufficiently), or when the system
has support for splitting live mappings.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/mm/pageattr.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index bbe98ac9ad8c..20ff9cb273c1 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -169,8 +169,6 @@ 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).
-	 * 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.
@@ -179,7 +177,16 @@ static int change_memory_common(unsigned long addr, int numpages,
 	if (!area ||
 	    ((unsigned long)kasan_reset_tag((void *)end) >
 	     (unsigned long)kasan_reset_tag(area->addr) + area->size) ||
-	    ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
+	    !(area->flags & VM_ALLOC))
+               return -EINVAL;
+
+	/*
+	 * Disallow VM_ALLOW_HUGE_VMAP mappings unless the region is PMD
+	 * aligned, or splitting live huge mappings is supported.
+	 */
+	if ((area->flags & VM_ALLOW_HUGE_VMAP) &&
+	   ((start % PMD_SIZE) || (size % PMD_SIZE)) &&
+	   WARN_ON_ONCE(!system_supports_bbml2_noabort()))
 		return -EINVAL;
 
 	if (!numpages)
-- 
2.55.0.860.g4b6b3295ed-goog