x86/vdso: Restrict splitting VVAR VMA

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.

Co-developed-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20191112012724.250792-20-dima@arista.com

diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
index f593774..76cbe54 100644 (file)
--- a/arch/x86/entry/vdso/vma.c
+++ b/arch/x86/entry/vdso/vma.c
@@ -84,6 +84,18 @@ static int vdso_mremap(const struct vm_special_mapping *sm,
        return 0;
 }
 
+static int vvar_mremap(const struct vm_special_mapping *sm,
+               struct vm_area_struct *new_vma)
+{
+       const struct vdso_image *image = new_vma->vm_mm->context.vdso_image;
+       unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
+
+       if (new_size != -image->sym_vvar_start)
+               return -EINVAL;
+
+       return 0;
+}
+
 static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
                      struct vm_area_struct *vma, struct vm_fault *vmf)
 {
@@ -136,6 +148,7 @@ static const struct vm_special_mapping vdso_mapping = {
 static const struct vm_special_mapping vvar_mapping = {
        .name = "[vvar]",
        .fault = vvar_fault,
+       .mremap = vvar_mremap,
 };
 
 /*