powerpc: add support for syscall stack randomization

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.

In order to avoid unconditional stack canaries on syscall entry (due to
the use of alloca()), also disable stack protector to avoid triggering
needless checks and slowing down the entry path. As there is no general
way to control stack protector coverage with a function attribute, this
must be disabled at the compilation unit level.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220701082435.126596-3-xiujianfeng@huawei.com

Author: Xiu Jianfeng

arch/powerpc/Kconfig

@@ -196,6 +196,7 @@ config PPC
 	select HAVE_ARCH_KASAN			if PPC_BOOK3E_64
 	select HAVE_ARCH_KASAN_VMALLOC		if HAVE_ARCH_KASAN
 	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

arch/powerpc/kernel/Makefile

@@ -54,6 +54,13 @@ CFLAGS_cputable.o += -DDISABLE_BRANCH_PROFILING
 CFLAGS_btext.o += -DDISABLE_BRANCH_PROFILING
 endif
 
+#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
+# Remove stack protector to avoid triggering unneeded stack canary
+# checks due to randomize_kstack_offset.
+CFLAGS_REMOVE_syscall.o = -fstack-protector -fstack-protector-strong
+CFLAGS_syscall.o += -fno-stack-protector
+#endif
+
 obj-y				:= cputable.o syscalls.o \
 				   irq.o align.o signal_$(BITS).o pmc.o vdso.o \
 				   process.o systbl.o idle.o \

arch/powerpc/kernel/syscall.c

@@ -2,6 +2,7 @@
 
 #include <linux/compat.h>
 #include <linux/context_tracking.h>
+#include <linux/randomize_kstack.h>
 
 #include <asm/interrupt.h>
 #include <asm/kup.h>
@@ -18,10 +19,12 @@ notrace long system_call_exception(long r3, long r4, long r5,
 				   long r6, long r7, long r8,
 				   unsigned long r0, struct pt_regs *regs)
 {
+	long ret;
 	syscall_fn f;
 
 	kuap_lock();
 
+	add_random_kstack_offset();
 	regs->orig_gpr3 = r3;
 
 	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
@@ -169,5 +172,19 @@ notrace long system_call_exception(long r3, long r4, long r5,
 		f = (void *)sys_call_table[r0];
 	}
 
-	return f(r3, r4, r5, r6, r7, r8);
+	ret = f(r3, r4, r5, r6, r7, r8);
+
+	/*
+	 * 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;
 }