vdso: Add generic time data storage

Historically each architecture defined their own way to store the vDSO
data page. Add a generic mechanism to provide storage for that page.

Furthermore this generic storage will be extended to also provide
uniform storage for *non*-time-related data, like the random state or
architecture-specific data. These will have their own pages and data
structures, so rename 'vdso_data' into 'vdso_time_data' to make that
split clear from the name.

Also introduce a new consistent naming scheme for the symbols related to
the vDSO, which makes it clear if the symbol is accessible from
userspace or kernel space and the type of data behind the symbol.

The generic fault handler contains an optimization to prefault the vvar
page when the timens page is accessed. This was lifted from s390 and x86.

Co-developed-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250204-vdso-store-rng-v3-5-13a4669dfc8c@linutronix.de

lib/vdso/Kconfig

@@ -43,3 +43,8 @@ config VDSO_GETRANDOM
 	bool
 	help
 	  Selected by architectures that support vDSO getrandom().
+
+config GENERIC_VDSO_DATA_STORE
+	bool
+	help
+	  Selected by architectures that use the generic vDSO data store.

lib/vdso/Makefile

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-$(CONFIG_GENERIC_VDSO_DATA_STORE) += datastore.o

lib/vdso/datastore.c

@@ -0,0 +1,103 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/linkage.h>
+#include <linux/mmap_lock.h>
+#include <linux/mm.h>
+#include <linux/time_namespace.h>
+#include <linux/types.h>
+#include <linux/vdso_datastore.h>
+#include <vdso/datapage.h>
+
+/*
+ * The vDSO data page.
+ */
+#ifdef CONFIG_HAVE_GENERIC_VDSO
+static union vdso_data_store vdso_time_data_store __page_aligned_data;
+struct vdso_time_data *vdso_k_time_data = vdso_time_data_store.data;
+static_assert(sizeof(vdso_time_data_store) == PAGE_SIZE);
+#endif /* CONFIG_HAVE_GENERIC_VDSO */
+
+static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
+			     struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	struct page *timens_page = find_timens_vvar_page(vma);
+	unsigned long addr, pfn;
+	vm_fault_t err;
+
+	switch (vmf->pgoff) {
+	case VDSO_TIME_PAGE_OFFSET:
+		if (!IS_ENABLED(CONFIG_HAVE_GENERIC_VDSO))
+			return VM_FAULT_SIGBUS;
+		pfn = __phys_to_pfn(__pa_symbol(vdso_k_time_data));
+		if (timens_page) {
+			/*
+			 * Fault in VVAR page too, since it will be accessed
+			 * to get clock data anyway.
+			 */
+			addr = vmf->address + VDSO_TIMENS_PAGE_OFFSET * PAGE_SIZE;
+			err = vmf_insert_pfn(vma, addr, pfn);
+			if (unlikely(err & VM_FAULT_ERROR))
+				return err;
+			pfn = page_to_pfn(timens_page);
+		}
+		break;
+	case VDSO_TIMENS_PAGE_OFFSET:
+		/*
+		 * If a task belongs to a time namespace then a namespace
+		 * specific VVAR is mapped with the VVAR_DATA_PAGE_OFFSET and
+		 * the real VVAR page is mapped with the VVAR_TIMENS_PAGE_OFFSET
+		 * offset.
+		 * See also the comment near timens_setup_vdso_data().
+		 */
+		if (!IS_ENABLED(CONFIG_TIME_NS) || !timens_page)
+			return VM_FAULT_SIGBUS;
+		pfn = __phys_to_pfn(__pa_symbol(vdso_k_time_data));
+		break;
+	default:
+		return VM_FAULT_SIGBUS;
+	}
+
+	return vmf_insert_pfn(vma, vmf->address, pfn);
+}
+
+const struct vm_special_mapping vdso_vvar_mapping = {
+	.name	= "[vvar]",
+	.fault	= vvar_fault,
+};
+
+struct vm_area_struct *vdso_install_vvar_mapping(struct mm_struct *mm, unsigned long addr)
+{
+	return _install_special_mapping(mm, addr, VDSO_NR_PAGES * PAGE_SIZE,
+					VM_READ | VM_MAYREAD | VM_IO | VM_DONTDUMP | VM_PFNMAP,
+					&vdso_vvar_mapping);
+}
+
+#ifdef CONFIG_TIME_NS
+/*
+ * The vvar page layout depends on whether a task belongs to the root or
+ * non-root time namespace. Whenever a task changes its namespace, the VVAR
+ * page tables are cleared and then they will be re-faulted with a
+ * corresponding layout.
+ * See also the comment near timens_setup_vdso_data() for details.
+ */
+int vdso_join_timens(struct task_struct *task, struct time_namespace *ns)
+{
+	struct mm_struct *mm = task->mm;
+	struct vm_area_struct *vma;
+	VMA_ITERATOR(vmi, mm, 0);
+
+	mmap_read_lock(mm);
+	for_each_vma(vmi, vma) {
+		if (vma_is_special_mapping(vma, &vdso_vvar_mapping))
+			zap_vma_pages(vma);
+	}
+	mmap_read_unlock(mm);
+
+	return 0;
+}
+
+struct vdso_time_data *arch_get_vdso_data(void *vvar_page)
+{
+	return (struct vdso_time_data *)vvar_page;
+}
+#endif

lib/vdso/gettimeofday.c

@@ -5,6 +5,9 @@
 #include <vdso/datapage.h>
 #include <vdso/helpers.h>
 
+/* Bring in default accessors */
+#include <vdso/vsyscall.h>
+
 #ifndef vdso_calc_ns
 
 #ifdef VDSO_DELTA_NOMASK
@@ -69,6 +72,16 @@ static inline bool vdso_cycles_ok(u64 cycles)
 #endif
 
 #ifdef CONFIG_TIME_NS
+
+#ifdef CONFIG_GENERIC_VDSO_DATA_STORE
+static __always_inline
+const struct vdso_time_data *__arch_get_vdso_u_timens_data(const struct vdso_time_data *vd)
+{
+	return (void *)vd + PAGE_SIZE;
+}
+#define __arch_get_timens_vdso_data(vd) __arch_get_vdso_u_timens_data(vd)
+#endif /* CONFIG_GENERIC_VDSO_DATA_STORE */
+
 static __always_inline int do_hres_timens(const struct vdso_data *vdns, clockid_t clk,
 					  struct __kernel_timespec *ts)
 {
@@ -282,7 +295,7 @@ __cvdso_clock_gettime_data(const struct vdso_data *vd, clockid_t clock,
 static __maybe_unused int
 __cvdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts)
 {
-	return __cvdso_clock_gettime_data(__arch_get_vdso_data(), clock, ts);
+	return __cvdso_clock_gettime_data(__arch_get_vdso_u_time_data(), clock, ts);
 }
 
 #ifdef BUILD_VDSO32
@@ -308,7 +321,7 @@ __cvdso_clock_gettime32_data(const struct vdso_data *vd, clockid_t clock,
 static __maybe_unused int
 __cvdso_clock_gettime32(clockid_t clock, struct old_timespec32 *res)
 {
-	return __cvdso_clock_gettime32_data(__arch_get_vdso_data(), clock, res);
+	return __cvdso_clock_gettime32_data(__arch_get_vdso_u_time_data(), clock, res);
 }
 #endif /* BUILD_VDSO32 */
 
@@ -342,7 +355,7 @@ __cvdso_gettimeofday_data(const struct vdso_data *vd,
 static __maybe_unused int
 __cvdso_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz)
 {
-	return __cvdso_gettimeofday_data(__arch_get_vdso_data(), tv, tz);
+	return __cvdso_gettimeofday_data(__arch_get_vdso_u_time_data(), tv, tz);
 }
 
 #ifdef VDSO_HAS_TIME
@@ -365,7 +378,7 @@ __cvdso_time_data(const struct vdso_data *vd, __kernel_old_time_t *time)
 
 static __maybe_unused __kernel_old_time_t __cvdso_time(__kernel_old_time_t *time)
 {
-	return __cvdso_time_data(__arch_get_vdso_data(), time);
+	return __cvdso_time_data(__arch_get_vdso_u_time_data(), time);
 }
 #endif /* VDSO_HAS_TIME */
 
@@ -425,7 +438,7 @@ int __cvdso_clock_getres_data(const struct vdso_data *vd, clockid_t clock,
 static __maybe_unused
 int __cvdso_clock_getres(clockid_t clock, struct __kernel_timespec *res)
 {
-	return __cvdso_clock_getres_data(__arch_get_vdso_data(), clock, res);
+	return __cvdso_clock_getres_data(__arch_get_vdso_u_time_data(), clock, res);
 }
 
 #ifdef BUILD_VDSO32
@@ -451,7 +464,7 @@ __cvdso_clock_getres_time32_data(const struct vdso_data *vd, clockid_t clock,
 static __maybe_unused int
 __cvdso_clock_getres_time32(clockid_t clock, struct old_timespec32 *res)
 {
-	return __cvdso_clock_getres_time32_data(__arch_get_vdso_data(),
+	return __cvdso_clock_getres_time32_data(__arch_get_vdso_u_time_data(),
 						clock, res);
 }
 #endif /* BUILD_VDSO32 */