arm64/vdso: use the fault callback to map vvar pages

Currently the vdso has no awareness of time namespaces, which may
apply distinct offsets to processes in different namespaces. To handle
this within the vdso, we'll need to expose a per-namespace data page.

As a preparatory step, this patch separates the vdso data page from
the code pages, and has it faulted in via its own fault callback.
Subsquent patches will extend this to support distinct pages per time
namespace.

The vvar vma has to be installed with the VM_PFNMAP flag to handle
faults via its vma fault callback.
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20200624083321.144975-2-avagin@gmail.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/vdso.c

@@ -107,29 +107,32 @@ static int __vdso_init(enum vdso_abi abi)
 			vdso_info[abi].vdso_code_start) >>
 			PAGE_SHIFT;
 
-	/* Allocate the vDSO pagelist, plus a page for the data. */
-	vdso_pagelist = kcalloc(vdso_info[abi].vdso_pages + 1,
+	vdso_pagelist = kcalloc(vdso_info[abi].vdso_pages,
 				sizeof(struct page *),
 				GFP_KERNEL);
 	if (vdso_pagelist == NULL)
 		return -ENOMEM;
 
-	/* Grab the vDSO data page. */
-	vdso_pagelist[0] = phys_to_page(__pa_symbol(vdso_data));
-
-
 	/* Grab the vDSO code pages. */
 	pfn = sym_to_pfn(vdso_info[abi].vdso_code_start);
 
 	for (i = 0; i < vdso_info[abi].vdso_pages; i++)
-		vdso_pagelist[i + 1] = pfn_to_page(pfn + i);
+		vdso_pagelist[i] = pfn_to_page(pfn + i);
 
-	vdso_info[abi].dm->pages = &vdso_pagelist[0];
-	vdso_info[abi].cm->pages = &vdso_pagelist[1];
+	vdso_info[abi].cm->pages = vdso_pagelist;
 
 	return 0;
 }
 
+static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
+			     struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	if (vmf->pgoff == 0)
+		return vmf_insert_pfn(vma, vmf->address,
+				sym_to_pfn(vdso_data));
+	return VM_FAULT_SIGBUS;
+}
+
 static int __setup_additional_pages(enum vdso_abi abi,
 				    struct mm_struct *mm,
 				    struct linux_binprm *bprm,
@@ -150,7 +153,7 @@ static int __setup_additional_pages(enum vdso_abi abi,
 	}
 
 	ret = _install_special_mapping(mm, vdso_base, PAGE_SIZE,
-				       VM_READ|VM_MAYREAD,
+				       VM_READ|VM_MAYREAD|VM_PFNMAP,
 				       vdso_info[abi].dm);
 	if (IS_ERR(ret))
 		goto up_fail;
@@ -206,6 +209,7 @@ static struct vm_special_mapping aarch32_vdso_maps[] = {
 #ifdef CONFIG_COMPAT_VDSO
 	[AA32_MAP_VVAR] = {
 		.name = "[vvar]",
+		.fault = vvar_fault,
 	},
 	[AA32_MAP_VDSO] = {
 		.name = "[vdso]",
@@ -371,6 +375,7 @@ enum aarch64_map {
 static struct vm_special_mapping aarch64_vdso_maps[] __ro_after_init = {
 	[AA64_MAP_VVAR] = {
 		.name	= "[vvar]",
+		.fault = vvar_fault,
 	},
 	[AA64_MAP_VDSO] = {
 		.name	= "[vdso]",