[tip: x86/core] x86/cpu: Re-enable stackprotector

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     2cb15faaedeb67f52f2ddc32b5ca152acfc422c2
Gitweb:        https://git.kernel.org/tip/2cb15faaedeb67f52f2ddc32b5ca152acfc422c2
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Thu, 15 Sep 2022 13:10:43 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 17 Oct 2022 16:40:56 +02:00

x86/cpu: Re-enable stackprotector

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@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220915111143.303010511@xxxxxxxxxxxxx
---
 arch/x86/kernel/cpu/Makefile | 3 ---
 arch/x86/kernel/cpu/common.c | 3 +++
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
index f10a921..d7e3cea 100644
--- a/arch/x86/kernel/cpu/Makefile
+++ b/arch/x86/kernel/cpu/Makefile
@@ -17,9 +17,6 @@ KMSAN_SANITIZE_common.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
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index f51928d..8e87318 100644
--- 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(int cpu)
 	 * 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