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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111143.303010511@infradead.org

diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
index f10a921..d7e3cea 100644 (file)
--- 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 (file)
--- 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