arm64: compat: Allow signal page to be remapped

For compatability with 32-bit Arm, allow the compat signal page to be
remapped via mremap().

Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210318170738.7756-4-will@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/vdso.c

@@ -271,6 +271,14 @@ enum aarch32_map {
 static struct page *aarch32_vectors_page __ro_after_init;
 static struct page *aarch32_sig_page __ro_after_init;
 
+static int aarch32_sigpage_mremap(const struct vm_special_mapping *sm,
+				  struct vm_area_struct *new_vma)
+{
+	current->mm->context.sigpage = (void *)new_vma->vm_start;
+
+	return 0;
+}
+
 static struct vm_special_mapping aarch32_vdso_maps[] = {
 	[AA32_MAP_VECTORS] = {
 		.name	= "[vectors]", /* ABI */
@@ -279,6 +287,7 @@ static struct vm_special_mapping aarch32_vdso_maps[] = {
 	[AA32_MAP_SIGPAGE] = {
 		.name	= "[sigpage]", /* ABI */
 		.pages	= &aarch32_sig_page,
+		.mremap	= aarch32_sigpage_mremap,
 	},
 	[AA32_MAP_VVAR] = {
 		.name = "[vvar]",