vdso: Add generic random data storage

Extend the generic vDSO data storage with a page for the random state data.
The random state data is stored in a dedicated page, as the existing
storage page is only meant for time-related, time-namespace-aware data.
This simplifies to access logic to not need to handle time namespaces
anymore and also frees up more space in the time-related page.

In case further generic vDSO data store is required it can be added to
the random state page.

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-v3-6-13a4669dfc8c@linutronix.de

Author: t-8ch

drivers/char/random.c

@@ -278,7 +278,7 @@ static void crng_reseed(struct work_struct *work)
 	WRITE_ONCE(base_crng.generation, next_gen);
 #ifdef CONFIG_VDSO_GETRANDOM
 	/* base_crng.generation's invalid value is ULONG_MAX, while
-	 * _vdso_rng_data.generation's invalid value is 0, so add one to the
+	 * vdso_k_rng_data->generation's invalid value is 0, so add one to the
 	 * former to arrive at the latter. Use smp_store_release so that this
 	 * is ordered with the write above to base_crng.generation. Pairs with
 	 * the smp_rmb() before the syscall in the vDSO code.
@@ -290,7 +290,7 @@ static void crng_reseed(struct work_struct *work)
 	 * because the vDSO side only checks whether the value changed, without
 	 * actually using or interpreting the value.
 	 */
-	smp_store_release((unsigned long *)&__arch_get_k_vdso_rng_data()->generation, next_gen + 1);
+	smp_store_release((unsigned long *)&vdso_k_rng_data->generation, next_gen + 1);
 #endif
 	if (!static_branch_likely(&crng_is_ready))
 		crng_init = CRNG_READY;
@@ -743,7 +743,7 @@ static void __cold _credit_init_bits(size_t bits)
 			queue_work(system_unbound_wq, &set_ready);
 		atomic_notifier_call_chain(&random_ready_notifier, 0, NULL);
 #ifdef CONFIG_VDSO_GETRANDOM
-		WRITE_ONCE(__arch_get_k_vdso_rng_data()->is_ready, true);
+		WRITE_ONCE(vdso_k_rng_data->is_ready, true);
 #endif
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);

include/asm-generic/vdso/vsyscall.h

@@ -13,6 +13,13 @@ static __always_inline const struct vdso_time_data *__arch_get_vdso_u_time_data(
 }
 #endif
 
+#ifndef __arch_get_vdso_u_rng_data
+static __always_inline const struct vdso_rng_data *__arch_get_vdso_u_rng_data(void)
+{
+	return &vdso_u_rng_data;
+}
+#endif
+
 #else  /* !CONFIG_GENERIC_VDSO_DATA_STORE */
 
 #ifndef __arch_get_k_vdso_data
@@ -25,6 +32,11 @@ static __always_inline struct vdso_data *__arch_get_k_vdso_data(void)
 
 #define __arch_get_vdso_u_time_data __arch_get_vdso_data
 
+#ifndef __arch_get_vdso_u_rng_data
+#define __arch_get_vdso_u_rng_data() __arch_get_vdso_rng_data()
+#endif
+#define vdso_k_rng_data __arch_get_k_vdso_rng_data()
+
 #endif /* CONFIG_GENERIC_VDSO_DATA_STORE */
 
 #ifndef __arch_update_vsyscall

include/vdso/datapage.h

@@ -144,8 +144,10 @@ extern struct vdso_time_data _timens_data[CS_BASES] __attribute__((visibility("h
 extern struct vdso_rng_data _vdso_rng_data __attribute__((visibility("hidden")));
 #else
 extern struct vdso_time_data vdso_u_time_data[CS_BASES] __attribute__((visibility("hidden")));
+extern struct vdso_rng_data vdso_u_rng_data __attribute__((visibility("hidden")));
 
 extern struct vdso_time_data *vdso_k_time_data;
+extern struct vdso_rng_data *vdso_k_rng_data;
 #endif
 
 /**
@@ -161,6 +163,7 @@ union vdso_data_store {
 enum vdso_pages {
 	VDSO_TIME_PAGE_OFFSET,
 	VDSO_TIMENS_PAGE_OFFSET,
+	VDSO_RNG_PAGE_OFFSET,
 	VDSO_NR_PAGES
 };
 
@@ -184,9 +187,16 @@ enum vdso_pages {
 
 #else /* !__ASSEMBLY__ */
 
+#ifdef CONFIG_VDSO_GETRANDOM
+#define __vdso_u_rng_data	PROVIDE(vdso_u_rng_data = vdso_u_data + 2 * PAGE_SIZE);
+#else
+#define __vdso_u_rng_data
+#endif
+
 #define VDSO_VVAR_SYMS						\
 	PROVIDE(vdso_u_data = . - __VDSO_PAGES * PAGE_SIZE);	\
 	PROVIDE(vdso_u_time_data = vdso_u_data);		\
+	__vdso_u_rng_data					\
 
 
 #endif /* !__ASSEMBLY__ */

lib/vdso/datastore.c

@@ -17,6 +17,15 @@ 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 */
 
+#ifdef CONFIG_VDSO_GETRANDOM
+static union {
+	struct vdso_rng_data	data;
+	u8			page[PAGE_SIZE];
+} vdso_rng_data_store __page_aligned_data;
+struct vdso_rng_data *vdso_k_rng_data = &vdso_rng_data_store.data;
+static_assert(sizeof(vdso_rng_data_store) == PAGE_SIZE);
+#endif /* CONFIG_VDSO_GETRANDOM */
+
 static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
 			     struct vm_area_struct *vma, struct vm_fault *vmf)
 {
@@ -53,6 +62,11 @@ static vm_fault_t vvar_fault(const struct vm_special_mapping *sm,
 			return VM_FAULT_SIGBUS;
 		pfn = __phys_to_pfn(__pa_symbol(vdso_k_time_data));
 		break;
+	case VDSO_RNG_PAGE_OFFSET:
+		if (!IS_ENABLED(CONFIG_VDSO_GETRANDOM))
+			return VM_FAULT_SIGBUS;
+		pfn = __phys_to_pfn(__pa_symbol(vdso_k_rng_data));
+		break;
 	default:
 		return VM_FAULT_SIGBUS;
 	}

lib/vdso/getrandom.c

@@ -12,6 +12,9 @@
 #include <uapi/linux/mman.h>
 #include <uapi/linux/random.h>
 
+/* Bring in default accessors */
+#include <vdso/vsyscall.h>
+
 #undef PAGE_SIZE
 #undef PAGE_MASK
 #define PAGE_SIZE (1UL << CONFIG_PAGE_SHIFT)
@@ -152,7 +155,7 @@ __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_
 
 		/*
 		 * Prevent the syscall from being reordered wrt current_generation. Pairs with the
-		 * smp_store_release(&_vdso_rng_data.generation) in random.c.
+		 * smp_store_release(&vdso_k_rng_data->generation) in random.c.
 		 */
 		smp_rmb();
 
@@ -256,5 +259,6 @@ __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_
 static __always_inline ssize_t
 __cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len)
 {
-	return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state, opaque_len);
+	return __cvdso_getrandom_data(__arch_get_vdso_u_rng_data(), buffer, len, flags,
+				      opaque_state, opaque_len);
 }