[PATCH v3 06/18] vdso: Add generic random data storage

Thomas Weißschuh
2025-02-04T12:06:11Z

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>
---
drivers/char/random.c | 6 +++---
include/asm-generic/vdso/vsyscall.h | 12 ++++++++++++
include/vdso/datapage.h | 10 ++++++++++
lib/vdso/datastore.c | 14 ++++++++++++++
lib/vdso/getrandom.c | 8 ++++++--
5 files changed, 45 insertions(+), 5 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 2581186fa61b7b278916a27f99ee22b4851dc37a..92cbd24a36d8c73ac6a17e8e787eeae3f6af1922 100644
--- a/drivers/char/random.c
+++ b/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);
diff --git a/include/asm-generic/vdso/vsyscall.h b/include/asm-generic/vdso/vsyscall.h
index ac5b93b91993224da245e80031b9f51e0c083f3c..a5f973e4e2723ef1815c88a7846f7c477e3febd9 100644
--- a/include/asm-generic/vdso/vsyscall.h
+++ b/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
diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index b3d8087488ff35fbbc4d5058ae21e2c6cc58ed9c..7dbc8797e3b8923c06f4b6b34790766e68b2abd2 100644
--- a/include/vdso/datapage.h
+++ b/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__ */
diff --git a/lib/vdso/datastore.c b/lib/vdso/datastore.c
index d3181768424cc3ee2e980bd537ac5ca14dfcb304..9260b00dc8520af109bd9dd05ac7f7504eafbea0 100644
--- a/lib/vdso/datastore.c
+++ b/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;
}
diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
index 938ca539aaa64bc46280ef6dd17aa661126699eb..440f8a6203a69a6462aafee4ad8d5670cef6a353 100644
--- a/lib/vdso/getrandom.c
+++ b/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);
}

--
2.48.1