entry: Provide [syscall_]enter_from_user_mode_randomize_stack()

Randomizing the syscall stack can only happen after state is established
via enter_from_user_mode() or syscall_enter_from_user_mode(). The earlier
it happens the better.

Provide two new macros to consolidate that:

  - enter_from_user_mode_randomize_stack()
	enter_from_user_mode();
	add_random_kstack_offset_irqsoff();

  - syscall_enter_from_user_mode_randomize_stack()
	enter_from_user_mode_randomize_stack();
	syscall_enter_from_user_mode_work();

to reduce boiler plate code.

Those are macros and not inline functions as the latter would limit the
stack randomization scope to the inline function itself.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Reviewed-by: Radu Rendec <radu@rendec.net>
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Link: https://patch.msgid.link/20260707190253.816918647@kernel.org

1 files changed, 56 insertions, 0 deletions

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index 9336516430a1..166b1ed9a9c0 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -6,6 +6,7 @@
 #include <linux/irq-entry-common.h>
 #include <linux/livepatch.h>
 #include <linux/ptrace.h>
+#include <linux/randomize_kstack.h>
 #include <linux/resume_user_mode.h>
 #include <linux/seccomp.h>
 #include <linux/sched.h>
@@ -150,6 +151,61 @@ static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *re
 }
 
 /**
+ * enter_from_user_mode_randomize_stack - Establish state and add stack randomization
+ *					  before invoking syscall_enter_from_user_mode_work()
+ * @regs:	Pointer to currents pt_regs
+ *
+ * Invoked from architecture specific syscall entry code with interrupts
+ * disabled. The calling code has to be non-instrumentable. When the function
+ * returns all state is correct, interrupts are still disabled and the
+ * subsequent functions can be instrumented.
+ *
+ * Implemented as a macro so that the stack randomization is effective
+ * throughout the function in which it is invoked. An inline would only make it
+ * effective in the scope of the inline function.
+ */
+#define enter_from_user_mode_randomize_stack(regs)			\
+do {									\
+	enter_from_user_mode(regs);					\
+	instrumentation_begin();					\
+	add_random_kstack_offset_irqsoff();				\
+	instrumentation_end();						\
+} while (0)
+
+/**
+ * syscall_enter_from_user_mode_randomize_stack - Establish state and check and handle work
+ *						  before invoking a syscall
+ * @regs:	Pointer to currents pt_regs
+ * @syscall:	The syscall number
+ *
+ * Invoked from architecture specific syscall entry code with interrupts
+ * disabled. The calling code has to be non-instrumentable. When the
+ * function returns all state is correct, interrupts are enabled and the
+ * subsequent functions can be instrumented.
+ *
+ * This is the combination of enter_from_user_mode_randomize_stack() and
+ * syscall_enter_from_user_mode_work() to be used when there is no
+ * architecture specific work to be done between the two.
+ *
+ * Returns: The original or a modified syscall number. See
+ * syscall_enter_from_user_mode_work() for further explanation.
+ *
+ * Implemented as a macro to make stack randomization effective in the calling
+ * scope.
+ */
+#define syscall_enter_from_user_mode_randomize_stack(regs, syscall)	\
+({									\
+	enter_from_user_mode_randomize_stack(regs);			\
+									\
+	instrumentation_begin();					\
+	local_irq_enable();						\
+	long _ret = syscall_enter_from_user_mode_work(regs, syscall);	\
+	instrumentation_end();						\
+									\
+	_ret;								\
+})
+
+/**
  * syscall_enter_from_user_mode - Establish state and check and handle work
  *				  before invoking a syscall
  * @regs:	Pointer to currents pt_regs