[PATCH v3 04/59] x86/cpu: Re-enable stackprotector

[Peter Zijlstra <peterz@infradead.org>]

From: Thomas Gleixner <tglx@linutronix.de>

Commit 5416c2663517 ("x86: make sure load_percpu_segment has no
stackprotector") disabled the stackprotector for cpu/common.c because of
load_percpu_segment(). Back then the boot stack canary was initialized very
early in start_kernel(). Switching the per CPU area by loading the GDT
caused the stackprotector to fail with paravirt enabled kernels as the
GSBASE was not updated yet. In hindsight a wrong change because it would
have been sufficient to ensure that the canary is the same in both per CPU
areas.

Commit d55535232c3d ("random: move rand_initialize() earlier") moved the
stack canary initialization to a later point in the init sequence. As a
consequence the per CPU stack canary is 0 when switching the per CPU areas,
so there is no requirement anymore to exclude this file.

Add a comment to load_percpu_segment().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/cpu/Makefile |    3 ---
 arch/x86/kernel/cpu/common.c |    3 +++
 2 files changed, 3 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/cpu/Makefile
+++ b/arch/x86/kernel/cpu/Makefile
@@ -16,9 +16,6 @@ KCOV_INSTRUMENT_perf_event.o := n
 # As above, instrumenting secondary CPU boot code causes boot hangs.
 KCSAN_SANITIZE_common.o := n
 
-# Make sure load_percpu_segment has no stackprotector
-CFLAGS_common.o		:= -fno-stack-protector
-
 obj-y			:= cacheinfo.o scattered.o topology.o
 obj-y			+= common.o
 obj-y			+= rdrand.o
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -752,6 +752,9 @@ void __init switch_gdt_and_percpu_base(i
 	 * early mapping is still valid. That means the GSBASE update will
 	 * lose any prior per CPU data which was not copied over in
 	 * setup_per_cpu_areas().
+	 *
+	 * This works even with stackprotector enabled because the
+	 * per CPU stack canary is 0 in both per CPU areas.
 	 */
 	wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
 #else