[PATCH 5/6] arm64/vdso: Restrict splitting VVAR VMA

[Andrei Vagin <avagin@gmail.com>]

Forbid splitting VVAR VMA resulting in a stricter ABI and reducing the
amount of corner-cases to consider while working further on VDSO time
namespace support.

As the offset from timens to VVAR page is computed compile-time, the pages
in VVAR should stay together and not being partically mremap()'ed.

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
---
 arch/arm64/kernel/vdso.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index f3baecd8edfb..8b17d7d10729 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -235,6 +235,17 @@ static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
 	return vmf_insert_pfn(vma, vmf->address, pfn);
 }
 
+static int vvar_mremap(const struct vm_special_mapping *sm,
+		       struct vm_area_struct *new_vma)
+{
+	unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
+
+	if (new_size != VVAR_NR_PAGES * PAGE_SIZE)
+		return -EINVAL;
+
+	return 0;
+}
+
 static int __setup_additional_pages(enum arch_vdso_type arch_index,
 				    struct mm_struct *mm,
 				    struct linux_binprm *bprm,
@@ -317,6 +328,7 @@ static struct vm_special_mapping aarch32_vdso_spec[C_PAGES] = {
 	{
 		.name = "[vvar]",
 		.fault = vvar_fault,
+		.mremap = vvar_mremap,
 	},
 	{
 		.name = "[vdso]",
@@ -488,6 +500,7 @@ static struct vm_special_mapping vdso_spec[A_PAGES] __ro_after_init = {
 	{
 		.name	= "[vvar]",
 		.fault = vvar_fault,
+		.mremap = vvar_mremap,
 	},
 	{
 		.name	= "[vdso]",
-- 
2.24.1