[PATCH -next] powerpc: add support for syscall stack randomization

[Xiu Jianfeng]

Add support for adding a random offset to the stack while handling
syscalls. This patch uses mftb() instead of get_random_int() for better
performance.

Signed-off-by: Xiu Jianfeng <xiujianfeng at huawei.com>
---
 arch/powerpc/Kconfig            | 1 +
 arch/powerpc/kernel/interrupt.c | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 5fc9153927ac..7e04c9f80cbc 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -192,6 +192,7 @@ config PPC
 	select HAVE_ARCH_KASAN			if PPC32 && PPC_PAGE_SHIFT <= 14
 	select HAVE_ARCH_KASAN_VMALLOC		if PPC32 && PPC_PAGE_SHIFT <= 14
 	select HAVE_ARCH_KFENCE			if PPC_BOOK3S_32 || PPC_8xx || 40x
+	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_MMAP_RND_BITS
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 784ea3289c84..459385769721 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -4,6 +4,7 @@
 #include <linux/err.h>
 #include <linux/compat.h>
 #include <linux/sched/debug.h> /* for show_regs */
+#include <linux/randomize_kstack.h>
 
 #include <asm/kup.h>
 #include <asm/cputime.h>
@@ -82,6 +83,7 @@ notrace long system_call_exception(long r3, long r4, long r5,
 
 	kuap_lock();
 
+	add_random_kstack_offset();
 	regs->orig_gpr3 = r3;
 
 	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
@@ -405,6 +407,7 @@ interrupt_exit_user_prepare_main(unsigned long ret, struct pt_regs *regs)
 
 	/* Restore user access locks last */
 	kuap_user_restore(regs);
+	choose_random_kstack_offset(mftb() & 0xFF);
 
 	return ret;
 }
-- 
2.17.1