[PATCH v3 0/3] Fix bugs and performance of kstack offset randomisation

[PATCH v3 0/3] Fix bugs and performance of kstack offset randomisation

[Ryan Roberts]

Hi All,

As I reported at [1], kstack offset randomisation suffers from a couple of bugs
and, on arm64 at least, the performance is poor. This series attempts to fix
both; patch 1 provides back-portable fixes for the functional bugs. Patches 2-3
propose a performance improvement approach.

I've looked at a few different options but ultimately decided that Jeremy's
original prng approach is the fastest. I made the argument that this approach is
secure "enough" in the RFC [2] and the responses indicated agreement.

More details in the commit logs.


Performance
===========

Mean and tail performance of 3 "small" syscalls was measured. syscall was made
10 million times and each individually measured and binned. These results have
low noise so I'm confident that they are trustworthy.

The baseline is v6.18-rc5 with stack randomization turned *off*. So I'm showing
performance cost of turning it on without any changes to the implementation,
then the reduced performance cost of turning it on with my changes applied.

**NOTE**: The below results were generated using the RFC patches but there is no
meaningful change, so the numbers are still valid.

arm64 (AWS Graviton3):
+-----------------+--------------+-------------+---------------+
| Benchmark       | Result Class |   v6.18-rc5 | per-task-prng |
|                 |              | rndstack-on |               |
|                 |              |             |               |
+=================+==============+=============+===============+
| syscall/getpid  | mean (ns)    |  (R) 15.62% |     (R) 3.43% |
|                 | p99 (ns)     | (R) 155.01% |     (R) 3.20% |
|                 | p99.9 (ns)   | (R) 156.71% |     (R) 2.93% |
+-----------------+--------------+-------------+---------------+
| syscall/getppid | mean (ns)    |  (R) 14.09% |     (R) 2.12% |
|                 | p99 (ns)     | (R) 152.81% |         1.55% |
|                 | p99.9 (ns)   | (R) 153.67% |         1.77% |
+-----------------+--------------+-------------+---------------+
| syscall/invalid | mean (ns)    |  (R) 13.89% |     (R) 3.32% |
|                 | p99 (ns)     | (R) 165.82% |     (R) 3.51% |
|                 | p99.9 (ns)   | (R) 168.83% |     (R) 3.77% |
+-----------------+--------------+-------------+---------------+

Because arm64 was previously using get_random_u16(), it was expensive when it
didn't have any buffered bits and had to call into the crng. That's what caused
the enormous tail latency.


x86 (AWS Sapphire Rapids):
+-----------------+--------------+-------------+---------------+
| Benchmark       | Result Class |   v6.18-rc5 | per-task-prng |
|                 |              | rndstack-on |               |
|                 |              |             |               |
+=================+==============+=============+===============+
| syscall/getpid  | mean (ns)    |  (R) 13.32% |     (R) 4.60% |
|                 | p99 (ns)     |  (R) 13.38% |    (R) 18.08% |
|                 | p99.9 (ns)   |      16.26% |    (R) 19.38% |
+-----------------+--------------+-------------+---------------+
| syscall/getppid | mean (ns)    |  (R) 11.96% |     (R) 5.26% |
|                 | p99 (ns)     |  (R) 11.83% |     (R) 8.35% |
|                 | p99.9 (ns)   |  (R) 11.42% |    (R) 22.37% |
+-----------------+--------------+-------------+---------------+
| syscall/invalid | mean (ns)    |  (R) 10.58% |     (R) 2.91% |
|                 | p99 (ns)     |  (R) 10.51% |     (R) 4.36% |
|                 | p99.9 (ns)   |  (R) 10.35% |    (R) 21.97% |
+-----------------+--------------+-------------+---------------+

I was surprised to see that the baseline cost on x86 is 10-12% since it is just
using rdtsc. But as I say, I believe the results are accurate.


Changes since v2 (RFC) [3]
==========================

- Moved late_initcall() to initialize kstack_rnd_state out of
  randomize_kstack.h and into main.c. (issue noticed by kernel test robot)

Changes since v1 (RFC) [2]
==========================

- Introduced patch 2 to make prandom_u32_state() __always_inline (needed since
  its called from noinstr code)
- In patch 3, prng is now per-cpu instead of per-task (per Ard)


[1] https://lore.kernel.org/all/dd8c37bc-795f-4c7a-9086-69e584d8ab24@arm.com/
[2] https://lore.kernel.org/all/20251127105958.2427758-1-ryan.roberts@arm.com/
[3] https://lore.kernel.org/all/20251215163520.1144179-1-ryan.roberts@arm.com/

Thanks,
Ryan


Ryan Roberts (3):
  randomize_kstack: Maintain kstack_offset per task
  prandom: Convert prandom_u32_state() to __always_inline
  randomize_kstack: Unify random source across arches

 arch/Kconfig                         |  5 ++-
 arch/arm64/kernel/syscall.c          | 11 ------
 arch/loongarch/kernel/syscall.c      | 11 ------
 arch/powerpc/kernel/syscall.c        | 12 -------
 arch/riscv/kernel/traps.c            | 12 -------
 arch/s390/include/asm/entry-common.h |  8 -----
 arch/x86/include/asm/entry-common.h  | 12 -------
 include/linux/prandom.h              | 19 +++++++++-
 include/linux/randomize_kstack.h     | 54 +++++++++++-----------------
 init/main.c                          |  9 ++++-
 kernel/fork.c                        |  1 +
 lib/random32.c                       | 19 ----------
 12 files changed, 49 insertions(+), 124 deletions(-)

--
2.43.0

[PATCH v3 1/3] randomize_kstack: Maintain kstack_offset per task

[Ryan Roberts]

kstack_offset was previously maintained per-cpu, but this caused a
couple of issues. So let's instead make it per-task.

Issue 1: add_random_kstack_offset() and choose_random_kstack_offset()
expected and required to be called with interrupts and preemption
disabled so that it could manipulate per-cpu state. But arm64, loongarch
and risc-v are calling them with interrupts and preemption enabled. I
don't _think_ this causes any functional issues, but it's certainly
unexpected and could lead to manipulating the wrong cpu's state, which
could cause a minor performance degradation due to bouncing the cache
lines. By maintaining the state per-task those functions can safely be
called in preemptible context.

Issue 2: add_random_kstack_offset() is called before executing the
syscall and expands the stack using a previously chosen rnadom offset.
choose_random_kstack_offset() is called after executing the syscall and
chooses and stores a new random offset for the next syscall. With
per-cpu storage for this offset, an attacker could force cpu migration
during the execution of the syscall and prevent the offset from being
updated for the original cpu such that it is predictable for the next
syscall on that cpu. By maintaining the state per-task, this problem
goes away because the per-task random offset is updated after the
syscall regardless of which cpu it is executing on.

Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Closes: https://lore.kernel.org/all/dd8c37bc-795f-4c7a-9086-69e584d8ab24@arm.com/
Cc: stable@vger.kernel.org
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 include/linux/randomize_kstack.h | 26 +++++++++++++++-----------
 include/linux/sched.h            |  4 ++++
 init/main.c                      |  1 -
 kernel/fork.c                    |  2 ++
 4 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index 1d982dbdd0d0..5d3916ca747c 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -9,7 +9,6 @@
 
 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
 			 randomize_kstack_offset);
-DECLARE_PER_CPU(u32, kstack_offset);
 
 /*
  * Do not use this anywhere else in the kernel. This is used here because
@@ -50,15 +49,14 @@ DECLARE_PER_CPU(u32, kstack_offset);
  * add_random_kstack_offset - Increase stack utilization by previously
  *			      chosen random offset
  *
- * This should be used in the syscall entry path when interrupts and
- * preempt are disabled, and after user registers have been stored to
- * the stack. For testing the resulting entropy, please see:
- * tools/testing/selftests/lkdtm/stack-entropy.sh
+ * This should be used in the syscall entry path after user registers have been
+ * stored to the stack. Preemption may be enabled. For testing the resulting
+ * entropy, please see: tools/testing/selftests/lkdtm/stack-entropy.sh
  */
 #define add_random_kstack_offset() do {					\
 	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
 				&randomize_kstack_offset)) {		\
-		u32 offset = raw_cpu_read(kstack_offset);		\
+		u32 offset = current->kstack_offset;			\
 		u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset));	\
 		/* Keep allocation even after "ptr" loses scope. */	\
 		asm volatile("" :: "r"(ptr) : "memory");		\
@@ -69,9 +67,9 @@ DECLARE_PER_CPU(u32, kstack_offset);
  * choose_random_kstack_offset - Choose the random offset for the next
  *				 add_random_kstack_offset()
  *
- * This should only be used during syscall exit when interrupts and
- * preempt are disabled. This position in the syscall flow is done to
- * frustrate attacks from userspace attempting to learn the next offset:
+ * This should only be used during syscall exit. Preemption may be enabled. This
+ * position in the syscall flow is done to frustrate attacks from userspace
+ * attempting to learn the next offset:
  * - Maximize the timing uncertainty visible from userspace: if the
  *   offset is chosen at syscall entry, userspace has much more control
  *   over the timing between choosing offsets. "How long will we be in
@@ -85,14 +83,20 @@ DECLARE_PER_CPU(u32, kstack_offset);
 #define choose_random_kstack_offset(rand) do {				\
 	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
 				&randomize_kstack_offset)) {		\
-		u32 offset = raw_cpu_read(kstack_offset);		\
+		u32 offset = current->kstack_offset;			\
 		offset = ror32(offset, 5) ^ (rand);			\
-		raw_cpu_write(kstack_offset, offset);			\
+		current->kstack_offset = offset;			\
 	}								\
 } while (0)
+
+static inline void random_kstack_task_init(struct task_struct *tsk)
+{
+	tsk->kstack_offset = 0;
+}
 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
 #define add_random_kstack_offset()		do { } while (0)
 #define choose_random_kstack_offset(rand)	do { } while (0)
+#define random_kstack_task_init(tsk)		do { } while (0)
 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
 
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d395f2810fac..9e0080ed1484 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1591,6 +1591,10 @@ struct task_struct {
 	unsigned long			prev_lowest_stack;
 #endif
 
+#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
+	u32				kstack_offset;
+#endif
+
 #ifdef CONFIG_X86_MCE
 	void __user			*mce_vaddr;
 	__u64				mce_kflags;
diff --git a/init/main.c b/init/main.c
index b84818ad9685..27fcbbde933e 100644
--- a/init/main.c
+++ b/init/main.c
@@ -830,7 +830,6 @@ static inline void initcall_debug_enable(void)
 #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
 DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
 			   randomize_kstack_offset);
-DEFINE_PER_CPU(u32, kstack_offset);
 
 static int __init early_randomize_kstack_offset(char *buf)
 {
diff --git a/kernel/fork.c b/kernel/fork.c
index b1f3915d5f8e..b061e1edbc43 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -95,6 +95,7 @@
 #include <linux/thread_info.h>
 #include <linux/kstack_erase.h>
 #include <linux/kasan.h>
+#include <linux/randomize_kstack.h>
 #include <linux/scs.h>
 #include <linux/io_uring.h>
 #include <linux/bpf.h>
@@ -2231,6 +2232,7 @@ __latent_entropy struct task_struct *copy_process(
 	if (retval)
 		goto bad_fork_cleanup_io;
 
+	random_kstack_task_init(p);
 	stackleak_task_init(p);
 
 	if (pid != &init_struct_pid) {
-- 
2.43.0

[PATCH v3 2/3] prandom: Convert prandom_u32_state() to __always_inline

[Ryan Roberts]

We will shortly use prandom_u32_state() to implement kstack offset
randomization and some arches need to call it from non-instrumentable
context. Given the function is just a handful of operations and doesn't
call out to any other functions, let's take the easy path and make it
__always_inline.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 include/linux/prandom.h | 19 ++++++++++++++++++-
 lib/random32.c          | 19 -------------------
 2 files changed, 18 insertions(+), 20 deletions(-)

diff --git a/include/linux/prandom.h b/include/linux/prandom.h
index ff7dcc3fa105..e797b3709f5c 100644
--- a/include/linux/prandom.h
+++ b/include/linux/prandom.h
@@ -17,7 +17,24 @@ struct rnd_state {
 	__u32 s1, s2, s3, s4;
 };
 
-u32 prandom_u32_state(struct rnd_state *state);
+/**
+ * prandom_u32_state - seeded pseudo-random number generator.
+ * @state: pointer to state structure holding seeded state.
+ *
+ * This is used for pseudo-randomness with no outside seeding.
+ * For more random results, use get_random_u32().
+ */
+static __always_inline u32 prandom_u32_state(struct rnd_state *state)
+{
+#define TAUSWORTHE(s, a, b, c, d) ((s & c) << d) ^ (((s << a) ^ s) >> b)
+	state->s1 = TAUSWORTHE(state->s1,  6U, 13U, 4294967294U, 18U);
+	state->s2 = TAUSWORTHE(state->s2,  2U, 27U, 4294967288U,  2U);
+	state->s3 = TAUSWORTHE(state->s3, 13U, 21U, 4294967280U,  7U);
+	state->s4 = TAUSWORTHE(state->s4,  3U, 12U, 4294967168U, 13U);
+
+	return (state->s1 ^ state->s2 ^ state->s3 ^ state->s4);
+}
+
 void prandom_bytes_state(struct rnd_state *state, void *buf, size_t nbytes);
 void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state);
 
diff --git a/lib/random32.c b/lib/random32.c
index 24e7acd9343f..d57baf489d4a 100644
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -42,25 +42,6 @@
 #include <linux/slab.h>
 #include <linux/unaligned.h>
 
-/**
- *	prandom_u32_state - seeded pseudo-random number generator.
- *	@state: pointer to state structure holding seeded state.
- *
- *	This is used for pseudo-randomness with no outside seeding.
- *	For more random results, use get_random_u32().
- */
-u32 prandom_u32_state(struct rnd_state *state)
-{
-#define TAUSWORTHE(s, a, b, c, d) ((s & c) << d) ^ (((s << a) ^ s) >> b)
-	state->s1 = TAUSWORTHE(state->s1,  6U, 13U, 4294967294U, 18U);
-	state->s2 = TAUSWORTHE(state->s2,  2U, 27U, 4294967288U,  2U);
-	state->s3 = TAUSWORTHE(state->s3, 13U, 21U, 4294967280U,  7U);
-	state->s4 = TAUSWORTHE(state->s4,  3U, 12U, 4294967168U, 13U);
-
-	return (state->s1 ^ state->s2 ^ state->s3 ^ state->s4);
-}
-EXPORT_SYMBOL(prandom_u32_state);
-
 /**
  *	prandom_bytes_state - get the requested number of pseudo-random bytes
  *
-- 
2.43.0

[PATCH v3 3/3] randomize_kstack: Unify random source across arches

[Ryan Roberts]

Previously different architectures were using random sources of
differing strength and cost to decide the random kstack offset. A number
of architectures (loongarch, powerpc, s390, x86) were using their
timestamp counter, at whatever the frequency happened to be. Other
arches (arm64, riscv) were using entropy from the crng via
get_random_u16().

There have been concerns that in some cases the timestamp counters may
be too weak, because they can be easily guessed or influenced by user
space. And get_random_u16() has been shown to be too costly for the
level of protection kstack offset randomization provides.

So let's use a common, architecture-agnostic source of entropy; a
per-cpu prng, seeded at boot-time from the crng. This has a few
benefits:

  - We can remove choose_random_kstack_offset(); That was only there to
    try to make the timestamp counter value a bit harder to influence
    from user space.

  - The architecture code is simplified. All it has to do now is call
    add_random_kstack_offset() in the syscall path.

  - The strength of the randomness can be reasoned about independently
    of the architecture.

  - Arches previously using get_random_u16() now have much faster
    syscall paths, see below results.

There have been some claims that a prng may be less strong than the
timestamp counter if not regularly reseeded. But the prng has a period
of about 2^113. So as long as the prng state remains secret, it should
not be possible to guess. If the prng state can be accessed, we have
bigger problems.

Additionally, we are only consuming 6 bits to randomize the stack, so
there are only 64 possible random offsets. I assert that it would be
trivial for an attacker to brute force by repeating their attack and
waiting for the random stack offset to be the desired one. The prng
approach seems entirely proportional to this level of protection.

Performance data are provided below. The baseline is v6.18 with rndstack
on for each respective arch. (I)/(R) indicate statistically significant
improvement/regression. arm64 platform is AWS Graviton3 (m7g.metal).
x86_64 platform is AWS Sapphire Rapids (m7i.24xlarge):

+-----------------+--------------+---------------+---------------+
| Benchmark       | Result Class | per-task-prng | per-task-prng |
|                 |              | arm64 (metal) |   x86_64 (VM) |
+=================+==============+===============+===============+
| syscall/getpid  | mean (ns)    |    (I) -9.50% |   (I) -17.65% |
|                 | p99 (ns)     |   (I) -59.24% |   (I) -24.41% |
|                 | p99.9 (ns)   |   (I) -59.52% |   (I) -28.52% |
+-----------------+--------------+---------------+---------------+
| syscall/getppid | mean (ns)    |    (I) -9.52% |   (I) -19.24% |
|                 | p99 (ns)     |   (I) -59.25% |   (I) -25.03% |
|                 | p99.9 (ns)   |   (I) -59.50% |   (I) -28.17% |
+-----------------+--------------+---------------+---------------+
| syscall/invalid | mean (ns)    |   (I) -10.31% |   (I) -18.56% |
|                 | p99 (ns)     |   (I) -60.79% |   (I) -20.06% |
|                 | p99.9 (ns)   |   (I) -61.04% |   (I) -25.04% |
+-----------------+--------------+---------------+---------------+

I tested an earlier version of this change on x86 bare metal and it
showed a smaller but still significant improvement. The bare metal
system wasn't available this time around so testing was done in a VM
instance. I'm guessing the cost of rdtsc is higher for VMs.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 arch/Kconfig                         |  5 ++-
 arch/arm64/kernel/syscall.c          | 11 ------
 arch/loongarch/kernel/syscall.c      | 11 ------
 arch/powerpc/kernel/syscall.c        | 12 -------
 arch/riscv/kernel/traps.c            | 12 -------
 arch/s390/include/asm/entry-common.h |  8 -----
 arch/x86/include/asm/entry-common.h  | 12 -------
 include/linux/randomize_kstack.h     | 52 +++++++++-------------------
 include/linux/sched.h                |  4 ---
 init/main.c                          |  8 +++++
 kernel/fork.c                        |  1 -
 11 files changed, 27 insertions(+), 109 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 31220f512b16..8591fe7b4ac1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1516,9 +1516,8 @@ config HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	def_bool n
 	help
 	  An arch should select this symbol if it can support kernel stack
-	  offset randomization with calls to add_random_kstack_offset()
-	  during syscall entry and choose_random_kstack_offset() during
-	  syscall exit. Careful removal of -fstack-protector-strong and
+	  offset randomization with a call to add_random_kstack_offset()
+	  during syscall entry. Careful removal of -fstack-protector-strong and
 	  -fstack-protector should also be applied to the entry code and
 	  closely examined, as the artificial stack bump looks like an array
 	  to the compiler, so it will attempt to add canary checks regardless
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index c062badd1a56..358ddfbf1401 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -52,17 +52,6 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
 	}
 
 	syscall_set_return_value(current, regs, 0, ret);
-
-	/*
-	 * This value will get limited by KSTACK_OFFSET_MAX(), which is 10
-	 * bits. The actual entropy will be further reduced by the compiler
-	 * when applying stack alignment constraints: the AAPCS mandates a
-	 * 16-byte aligned SP at function boundaries, which will remove the
-	 * 4 low bits from any entropy chosen here.
-	 *
-	 * The resulting 6 bits of entropy is seen in SP[9:4].
-	 */
-	choose_random_kstack_offset(get_random_u16());
 }
 
 static inline bool has_syscall_work(unsigned long flags)
diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/syscall.c
index 1249d82c1cd0..85da7e050d97 100644
--- a/arch/loongarch/kernel/syscall.c
+++ b/arch/loongarch/kernel/syscall.c
@@ -79,16 +79,5 @@ void noinstr __no_stack_protector do_syscall(struct pt_regs *regs)
 					   regs->regs[7], regs->regs[8], regs->regs[9]);
 	}
 
-	/*
-	 * This value will get limited by KSTACK_OFFSET_MAX(), which is 10
-	 * bits. The actual entropy will be further reduced by the compiler
-	 * when applying stack alignment constraints: 16-bytes (i.e. 4-bits)
-	 * aligned, which will remove the 4 low bits from any entropy chosen
-	 * here.
-	 *
-	 * The resulting 6 bits of entropy is seen in SP[9:4].
-	 */
-	choose_random_kstack_offset(get_cycles());
-
 	syscall_exit_to_user_mode(regs);
 }
diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index be159ad4b77b..b3d8b0f9823b 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c
@@ -173,17 +173,5 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
 	}
 #endif
 
-	/*
-	 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
-	 * so the maximum stack offset is 1k bytes (10 bits).
-	 *
-	 * The actual entropy will be further reduced by the compiler when
-	 * applying stack alignment constraints: the powerpc architecture
-	 * may have two kinds of stack alignment (16-bytes and 8-bytes).
-	 *
-	 * So the resulting 6 or 7 bits of entropy is seen in SP[9:4] or SP[9:3].
-	 */
-	choose_random_kstack_offset(mftb());
-
 	return ret;
 }
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 80230de167de..79b285bdfd1a 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -342,18 +342,6 @@ void do_trap_ecall_u(struct pt_regs *regs)
 		if (syscall >= 0 && syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
 
-		/*
-		 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
-		 * so the maximum stack offset is 1k bytes (10 bits).
-		 *
-		 * The actual entropy will be further reduced by the compiler when
-		 * applying stack alignment constraints: 16-byte (i.e. 4-bit) aligned
-		 * for RV32I or RV64I.
-		 *
-		 * The resulting 6 bits of entropy is seen in SP[9:4].
-		 */
-		choose_random_kstack_offset(get_random_u16());
-
 		syscall_exit_to_user_mode(regs);
 	} else {
 		irqentry_state_t state = irqentry_nmi_enter(regs);
diff --git a/arch/s390/include/asm/entry-common.h b/arch/s390/include/asm/entry-common.h
index 979af986a8fe..35450a485323 100644
--- a/arch/s390/include/asm/entry-common.h
+++ b/arch/s390/include/asm/entry-common.h
@@ -51,14 +51,6 @@ static __always_inline void arch_exit_to_user_mode(void)
 
 #define arch_exit_to_user_mode arch_exit_to_user_mode
 
-static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
-						  unsigned long ti_work)
-{
-	choose_random_kstack_offset(get_tod_clock_fast());
-}
-
-#define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare
-
 static __always_inline bool arch_in_rcu_eqs(void)
 {
 	if (IS_ENABLED(CONFIG_KVM))
diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index ce3eb6d5fdf9..7535131c711b 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -82,18 +82,6 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 	current_thread_info()->status &= ~(TS_COMPAT | TS_I386_REGS_POKED);
 #endif
 
-	/*
-	 * This value will get limited by KSTACK_OFFSET_MAX(), which is 10
-	 * bits. The actual entropy will be further reduced by the compiler
-	 * when applying stack alignment constraints (see cc_stack_align4/8 in
-	 * arch/x86/Makefile), which will remove the 3 (x86_64) or 2 (ia32)
-	 * low bits from any entropy chosen here.
-	 *
-	 * Therefore, final stack offset entropy will be 7 (x86_64) or
-	 * 8 (ia32) bits.
-	 */
-	choose_random_kstack_offset(rdtsc());
-
 	/* Avoid unnecessary reads of 'x86_ibpb_exit_to_user' */
 	if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER) &&
 	    this_cpu_read(x86_ibpb_exit_to_user)) {
diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index 5d3916ca747c..024fc20e7762 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -6,6 +6,7 @@
 #include <linux/kernel.h>
 #include <linux/jump_label.h>
 #include <linux/percpu-defs.h>
+#include <linux/prandom.h>
 
 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
 			 randomize_kstack_offset);
@@ -45,9 +46,22 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
 #define KSTACK_OFFSET_MAX(x)	((x) & 0b1111111100)
 #endif
 
+DECLARE_PER_CPU(struct rnd_state, kstack_rnd_state);
+
+static __always_inline u32 get_kstack_offset(void)
+{
+	struct rnd_state *state;
+	u32 rnd;
+
+	state = &get_cpu_var(kstack_rnd_state);
+	rnd = prandom_u32_state(state);
+	put_cpu_var(kstack_rnd_state);
+
+	return rnd;
+}
+
 /**
- * add_random_kstack_offset - Increase stack utilization by previously
- *			      chosen random offset
+ * add_random_kstack_offset - Increase stack utilization by a random offset.
  *
  * This should be used in the syscall entry path after user registers have been
  * stored to the stack. Preemption may be enabled. For testing the resulting
@@ -56,47 +70,15 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
 #define add_random_kstack_offset() do {					\
 	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
 				&randomize_kstack_offset)) {		\
-		u32 offset = current->kstack_offset;			\
+		u32 offset = get_kstack_offset();			\
 		u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset));	\
 		/* Keep allocation even after "ptr" loses scope. */	\
 		asm volatile("" :: "r"(ptr) : "memory");		\
 	}								\
 } while (0)
 
-/**
- * choose_random_kstack_offset - Choose the random offset for the next
- *				 add_random_kstack_offset()
- *
- * This should only be used during syscall exit. Preemption may be enabled. This
- * position in the syscall flow is done to frustrate attacks from userspace
- * attempting to learn the next offset:
- * - Maximize the timing uncertainty visible from userspace: if the
- *   offset is chosen at syscall entry, userspace has much more control
- *   over the timing between choosing offsets. "How long will we be in
- *   kernel mode?" tends to be more difficult to predict than "how long
- *   will we be in user mode?"
- * - Reduce the lifetime of the new offset sitting in memory during
- *   kernel mode execution. Exposure of "thread-local" memory content
- *   (e.g. current, percpu, etc) tends to be easier than arbitrary
- *   location memory exposure.
- */
-#define choose_random_kstack_offset(rand) do {				\
-	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
-				&randomize_kstack_offset)) {		\
-		u32 offset = current->kstack_offset;			\
-		offset = ror32(offset, 5) ^ (rand);			\
-		current->kstack_offset = offset;			\
-	}								\
-} while (0)
-
-static inline void random_kstack_task_init(struct task_struct *tsk)
-{
-	tsk->kstack_offset = 0;
-}
 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
 #define add_random_kstack_offset()		do { } while (0)
-#define choose_random_kstack_offset(rand)	do { } while (0)
-#define random_kstack_task_init(tsk)		do { } while (0)
 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
 
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9e0080ed1484..d395f2810fac 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1591,10 +1591,6 @@ struct task_struct {
 	unsigned long			prev_lowest_stack;
 #endif
 
-#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
-	u32				kstack_offset;
-#endif
-
 #ifdef CONFIG_X86_MCE
 	void __user			*mce_vaddr;
 	__u64				mce_kflags;
diff --git a/init/main.c b/init/main.c
index 27fcbbde933e..8626e048095a 100644
--- a/init/main.c
+++ b/init/main.c
@@ -830,6 +830,14 @@ static inline void initcall_debug_enable(void)
 #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
 DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
 			   randomize_kstack_offset);
+DEFINE_PER_CPU(struct rnd_state, kstack_rnd_state);
+
+static int __init random_kstack_init(void)
+{
+	prandom_seed_full_state(&kstack_rnd_state);
+	return 0;
+}
+late_initcall(random_kstack_init);
 
 static int __init early_randomize_kstack_offset(char *buf)
 {
diff --git a/kernel/fork.c b/kernel/fork.c
index b061e1edbc43..68d9766288fd 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2232,7 +2232,6 @@ __latent_entropy struct task_struct *copy_process(
 	if (retval)
 		goto bad_fork_cleanup_io;
 
-	random_kstack_task_init(p);
 	stackleak_task_init(p);
 
 	if (pid != &init_struct_pid) {
-- 
2.43.0

Re: [PATCH v3 2/3] prandom: Convert prandom_u32_state() to __always_inline

[Jason A. Donenfeld]

Hi Ryan,

On Fri, Jan 2, 2026 at 2:12 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> context. Given the function is just a handful of operations and doesn't

How many? What's this looking like in terms of assembly? It'd also be
nice to have some brief analysis of other call sites to have
confirmation this isn't blowing up other users.

> +static __always_inline u32 prandom_u32_state(struct rnd_state *state)

Why not just normal `inline`? Is gcc disagreeing with the inlinability
of this function?

Jason

Re: [PATCH v3 2/3] prandom: Convert prandom_u32_state() to __always_inline

[Ryan Roberts]

On 02/01/2026 13:39, Jason A. Donenfeld wrote:
> Hi Ryan,
> 
> On Fri, Jan 2, 2026 at 2:12 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>> context. Given the function is just a handful of operations and doesn't
> 
> How many? What's this looking like in terms of assembly? 

25 instructions on arm64:

0000000000000000 <prandom_u32_state>:
   0:	29401403 	ldp	w3, w5, [x0]
   4:	aa0003e1 	mov	x1, x0
   8:	29410002 	ldp	w2, w0, [x0, #8]
   c:	531e74a4 	lsl	w4, w5, #2
  10:	530e3468 	lsl	w8, w3, #18
  14:	4a0400a5 	eor	w5, w5, w4
  18:	4a031863 	eor	w3, w3, w3, lsl #6
  1c:	53196047 	lsl	w7, w2, #7
  20:	53134806 	lsl	w6, w0, #13
  24:	4a023442 	eor	w2, w2, w2, lsl #13
  28:	4a000c00 	eor	w0, w0, w0, lsl #3
  2c:	121b6884 	and	w4, w4, #0xffffffe0
  30:	120d3108 	and	w8, w8, #0xfff80000
  34:	121550e7 	and	w7, w7, #0xfffff800
  38:	120c2cc6 	and	w6, w6, #0xfff00000
  3c:	2a456c85 	orr	w5, w4, w5, lsr #27
  40:	2a433504 	orr	w4, w8, w3, lsr #13
  44:	2a4254e3 	orr	w3, w7, w2, lsr #21
  48:	2a4030c2 	orr	w2, w6, w0, lsr #12
  4c:	4a020066 	eor	w6, w3, w2
  50:	4a050080 	eor	w0, w4, w5
  54:	4a0000c0 	eor	w0, w6, w0
  58:	29001424 	stp	w4, w5, [x1]
  5c:	29010823 	stp	w3, w2, [x1, #8]
  60:	d65f03c0 	ret

> It'd also be
> nice to have some brief analysis of other call sites to have
> confirmation this isn't blowing up other users.

I compiled defconfig before and after this patch on arm64 and compared the text
sizes:

$ ./scripts/bloat-o-meter -t vmlinux.before vmlinux.after
add/remove: 3/4 grow/shrink: 4/1 up/down: 836/-128 (708)
Function                                     old     new   delta
prandom_seed_full_state                      364     932    +568
pick_next_task_fair                         1940    2036     +96
bpf_user_rnd_u32                             104     196     +92
prandom_bytes_state                          204     260     +56
e843419@0f2b_00012d69_e34                      -       8      +8
e843419@0db7_00010ec3_23ec                     -       8      +8
e843419@02cb_00003767_25c                      -       8      +8
bpf_prog_select_runtime                      448     444      -4
e843419@0aa3_0000cfd1_1580                     8       -      -8
e843419@0aa2_0000cfba_147c                     8       -      -8
e843419@075f_00008d8c_184                      8       -      -8
prandom_u32_state                            100       -    -100
Total: Before=19078072, After=19078780, chg +0.00%

So 708 bytes more after inlining. The main cost is prandom_seed_full_state(),
which calls prandom_u32_state() 10 times (via prandom_warmup()). I expect we
could turn that into a loop to reduce ~450 bytes overall.

I'm not really sure if 708 is good or bad...

> 
>> +static __always_inline u32 prandom_u32_state(struct rnd_state *state)
> 
> Why not just normal `inline`? Is gcc disagreeing with the inlinability
> of this function?

Given this needs to be called from a noinstr function, I didn't want to give the
compiler the opportunity to decide not to inline it, since in that case, some
instrumentation might end up being applied to the function body which would blow
up when called in the noinstr context.

I think the other 2 options are to keep prandom_u32_state() in the c file but
mark it noinstr or rearrange all the users so that thay don't call it until
instrumentation is allowable. The latter is something I was trying to avoid.

There is some previous discussion of this at [1].

[1] https://lore.kernel.org/all/aS65LFUfdgRPKv1l@J2N7QTR9R3/

Perhaps keeping prandom_u32_state() in the c file and making it noinstr is the
best compromise?

Thanks,
Ryan

> 
> Jason

Re: [PATCH v3 1/3] randomize_kstack: Maintain kstack_offset per task

[David Laight]

On Fri,  2 Jan 2026 13:11:52 +0000
Ryan Roberts <ryan.roberts@arm.com> wrote:

> kstack_offset was previously maintained per-cpu, but this caused a
> couple of issues. So let's instead make it per-task.
> 
> Issue 1: add_random_kstack_offset() and choose_random_kstack_offset()
> expected and required to be called with interrupts and preemption
> disabled so that it could manipulate per-cpu state. But arm64, loongarch
> and risc-v are calling them with interrupts and preemption enabled. I
> don't _think_ this causes any functional issues, but it's certainly
> unexpected and could lead to manipulating the wrong cpu's state, which
> could cause a minor performance degradation due to bouncing the cache
> lines. By maintaining the state per-task those functions can safely be
> called in preemptible context.
> 
> Issue 2: add_random_kstack_offset() is called before executing the
> syscall and expands the stack using a previously chosen rnadom offset.
                                                           <>
	David

> choose_random_kstack_offset() is called after executing the syscall and
> chooses and stores a new random offset for the next syscall. With
> per-cpu storage for this offset, an attacker could force cpu migration
> during the execution of the syscall and prevent the offset from being
> updated for the original cpu such that it is predictable for the next
> syscall on that cpu. By maintaining the state per-task, this problem
> goes away because the per-task random offset is updated after the
> syscall regardless of which cpu it is executing on.
> 
> Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
> Closes: https://lore.kernel.org/all/dd8c37bc-795f-4c7a-9086-69e584d8ab24@arm.com/
> Cc: stable@vger.kernel.org
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  include/linux/randomize_kstack.h | 26 +++++++++++++++-----------
>  include/linux/sched.h            |  4 ++++
>  init/main.c                      |  1 -
>  kernel/fork.c                    |  2 ++
>  4 files changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> index 1d982dbdd0d0..5d3916ca747c 100644
> --- a/include/linux/randomize_kstack.h
> +++ b/include/linux/randomize_kstack.h
> @@ -9,7 +9,6 @@
>  
>  DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
>  			 randomize_kstack_offset);
> -DECLARE_PER_CPU(u32, kstack_offset);
>  
>  /*
>   * Do not use this anywhere else in the kernel. This is used here because
> @@ -50,15 +49,14 @@ DECLARE_PER_CPU(u32, kstack_offset);
>   * add_random_kstack_offset - Increase stack utilization by previously
>   *			      chosen random offset
>   *
> - * This should be used in the syscall entry path when interrupts and
> - * preempt are disabled, and after user registers have been stored to
> - * the stack. For testing the resulting entropy, please see:
> - * tools/testing/selftests/lkdtm/stack-entropy.sh
> + * This should be used in the syscall entry path after user registers have been
> + * stored to the stack. Preemption may be enabled. For testing the resulting
> + * entropy, please see: tools/testing/selftests/lkdtm/stack-entropy.sh
>   */
>  #define add_random_kstack_offset() do {					\
>  	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
>  				&randomize_kstack_offset)) {		\
> -		u32 offset = raw_cpu_read(kstack_offset);		\
> +		u32 offset = current->kstack_offset;			\
>  		u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset));	\
>  		/* Keep allocation even after "ptr" loses scope. */	\
>  		asm volatile("" :: "r"(ptr) : "memory");		\
> @@ -69,9 +67,9 @@ DECLARE_PER_CPU(u32, kstack_offset);
>   * choose_random_kstack_offset - Choose the random offset for the next
>   *				 add_random_kstack_offset()
>   *
> - * This should only be used during syscall exit when interrupts and
> - * preempt are disabled. This position in the syscall flow is done to
> - * frustrate attacks from userspace attempting to learn the next offset:
> + * This should only be used during syscall exit. Preemption may be enabled. This
> + * position in the syscall flow is done to frustrate attacks from userspace
> + * attempting to learn the next offset:
>   * - Maximize the timing uncertainty visible from userspace: if the
>   *   offset is chosen at syscall entry, userspace has much more control
>   *   over the timing between choosing offsets. "How long will we be in
> @@ -85,14 +83,20 @@ DECLARE_PER_CPU(u32, kstack_offset);
>  #define choose_random_kstack_offset(rand) do {				\
>  	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
>  				&randomize_kstack_offset)) {		\
> -		u32 offset = raw_cpu_read(kstack_offset);		\
> +		u32 offset = current->kstack_offset;			\
>  		offset = ror32(offset, 5) ^ (rand);			\
> -		raw_cpu_write(kstack_offset, offset);			\
> +		current->kstack_offset = offset;			\
>  	}								\
>  } while (0)
> +
> +static inline void random_kstack_task_init(struct task_struct *tsk)
> +{
> +	tsk->kstack_offset = 0;
> +}
>  #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
>  #define add_random_kstack_offset()		do { } while (0)
>  #define choose_random_kstack_offset(rand)	do { } while (0)
> +#define random_kstack_task_init(tsk)		do { } while (0)
>  #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
>  
>  #endif
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index d395f2810fac..9e0080ed1484 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1591,6 +1591,10 @@ struct task_struct {
>  	unsigned long			prev_lowest_stack;
>  #endif
>  
> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
> +	u32				kstack_offset;
> +#endif
> +
>  #ifdef CONFIG_X86_MCE
>  	void __user			*mce_vaddr;
>  	__u64				mce_kflags;
> diff --git a/init/main.c b/init/main.c
> index b84818ad9685..27fcbbde933e 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -830,7 +830,6 @@ static inline void initcall_debug_enable(void)
>  #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
>  DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
>  			   randomize_kstack_offset);
> -DEFINE_PER_CPU(u32, kstack_offset);
>  
>  static int __init early_randomize_kstack_offset(char *buf)
>  {
> diff --git a/kernel/fork.c b/kernel/fork.c
> index b1f3915d5f8e..b061e1edbc43 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -95,6 +95,7 @@
>  #include <linux/thread_info.h>
>  #include <linux/kstack_erase.h>
>  #include <linux/kasan.h>
> +#include <linux/randomize_kstack.h>
>  #include <linux/scs.h>
>  #include <linux/io_uring.h>
>  #include <linux/bpf.h>
> @@ -2231,6 +2232,7 @@ __latent_entropy struct task_struct *copy_process(
>  	if (retval)
>  		goto bad_fork_cleanup_io;
>  
> +	random_kstack_task_init(p);
>  	stackleak_task_init(p);
>  
>  	if (pid != &init_struct_pid) {

Re: [PATCH v3 2/3] prandom: Convert prandom_u32_state() to __always_inline

[David Laight]

On Fri, 2 Jan 2026 14:39:21 +0100
"Jason A. Donenfeld" <Jason@zx2c4.com> wrote:

> Hi Ryan,
...
> > +static __always_inline u32 prandom_u32_state(struct rnd_state *state)  
> 
> Why not just normal `inline`? Is gcc disagreeing with the inlinability
> of this function?

gcc has a mind of its own when it comes to inlining.
If there weren't some massive functions marked 'inline' that should never
really be inlined then making 'inline' '__always_inline' would make sense.
But first an audit would be needed.
(This has come up several times in the past.)
But if you need a function to be inlined (for any reason) it needs to be
always_inline.

Whether there should be an non-inlined 'option' here is another matter.
There could be a normal function that calls the inlined version.

	David


> 
> Jason
>