randomize_kstack: Unify random source across arches

ryanhrob · kees · commit a96ef5848cb0 · 2026-03-24T21:12:03.000-07:00
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. [*] Additionally, this gets rid of some redundant work on s390 and x86. Before this patch, those architectures called choose_random_kstack_offset() under arch_exit_to_user_mode_prepare(), which is also called for exception returns to userspace which were *not* syscalls (e.g. regular interrupts). Getting rid of choose_random_kstack_offset() avoids a small amount of redundant work for the non-syscall cases. In some configurations, add_random_kstack_offset() will now call instrumentable code, so for a couple of arches, I have moved the call a bit later to the first point where instrumentation is allowed. This doesn't impact the efficacy of the mechanism. 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-cpu-prng | per-cpu-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. Acked-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> Link: https://patch.msgid.link/20260303150840.3789438-3-ryan.roberts@arm.com Signed-off-by: Kees Cook <kees@kernel.org>
diff --git a/arch/Kconfig b/arch/Kconfig
@@ -1519,9 +1519,8 @@ 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
@@ -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
@@ -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
@@ -20,8 +20,6 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
 
 	kuap_lock();
 
-	add_random_kstack_offset();
-
 	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
 		BUG_ON(irq_soft_mask_return() != IRQS_ALL_DISABLED);
 
@@ -30,6 +28,8 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
 	CT_WARN_ON(ct_state() == CT_STATE_KERNEL);
 	user_exit_irqoff();
 
+	add_random_kstack_offset();
+
 	BUG_ON(regs_is_unrecoverable(regs));
 	BUG_ON(!user_mode(regs));
 	BUG_ON(arch_irq_disabled_regs(regs));
@@ -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
@@ -344,18 +344,6 @@ void do_trap_ecall_u(struct pt_regs *regs)
 			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
@@ -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/s390/kernel/syscall.c b/arch/s390/kernel/syscall.c
@@ -96,8 +96,8 @@ void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
 {
 	unsigned long nr;
 
-	add_random_kstack_offset();
 	enter_from_user_mode(regs);
+	add_random_kstack_offset();
 	regs->psw = get_lowcore()->svc_old_psw;
 	regs->int_code = get_lowcore()->svc_int_code;
 	update_timer_sys();
diff --git a/arch/x86/entry/syscall_32.c b/arch/x86/entry/syscall_32.c
@@ -247,7 +247,6 @@ __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
 {
 	int nr = syscall_32_enter(regs);
 
-	add_random_kstack_offset();
 	/*
 	 * Subtlety here: if ptrace pokes something larger than 2^31-1 into
 	 * orig_ax, the int return value truncates it. This matches
@@ -256,6 +255,7 @@ __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
 	nr = syscall_enter_from_user_mode(regs, nr);
 	instrumentation_begin();
 
+	add_random_kstack_offset();
 	do_syscall_32_irqs_on(regs, nr);
 
 	instrumentation_end();
@@ -268,7 +268,6 @@ static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
 	int nr = syscall_32_enter(regs);
 	int res;
 
-	add_random_kstack_offset();
 	/*
 	 * This cannot use syscall_enter_from_user_mode() as it has to
 	 * fetch EBP before invoking any of the syscall entry work
@@ -277,6 +276,7 @@ static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
 	enter_from_user_mode(regs);
 
 	instrumentation_begin();
+	add_random_kstack_offset();
 	local_irq_enable();
 	/* Fetch EBP from where the vDSO stashed it. */
 	if (IS_ENABLED(CONFIG_X86_64)) {
diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
@@ -86,10 +86,10 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
 /* Returns true to return using SYSRET, or false to use IRET */
 __visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
 {
-	add_random_kstack_offset();
 	nr = syscall_enter_from_user_mode(regs, nr);
 
 	instrumentation_begin();
+	add_random_kstack_offset();
 
 	if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
 		/* Invalid system call, but still a system call. */
diff --git 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
@@ -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
@@ -1592,10 +1592,6 @@ 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
@@ -833,6 +833,14 @@ 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
@@ -2234,7 +2234,6 @@ __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) {

Original file line number	Diff line number	Diff line change
`@@ -96,8 +96,8 @@ void noinstr __do_syscall(struct pt_regs *regs, int per_trap)`
`96`	`96`	`{`
`97`	`97`	`unsigned long nr;`
`98`	`98`
`99`		`- add_random_kstack_offset();`
`100`	`99`	`enter_from_user_mode(regs);`
	`100`	`+ add_random_kstack_offset();`
`101`	`101`	`regs->psw = get_lowcore()->svc_old_psw;`
`102`	`102`	`regs->int_code = get_lowcore()->svc_int_code;`
`103`	`103`	`update_timer_sys();`