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Subject[PATCH v6 25/42] x86/head: re-enable stack protection for 32/64-bit builds
From: Michael Roth <>

As of commit 103a4908ad4d ("x86/head/64: Disable stack protection for
head$(BITS).o") kernel/head64.c is compiled with -fno-stack-protector
to allow a call to set_bringup_idt_handler(), which would otherwise
have stack protection enabled with CONFIG_STACKPROTECTOR_STRONG. While
sufficient for that case, there may still be issues with calls to any
external functions that were compiled with stack protection enabled that
in-turn make stack-protected calls, or if the exception handlers set up
by set_bringup_idt_handler() make calls to stack-protected functions.
As part of 103a4908ad4d, stack protection was also disabled for
kernel/head32.c as a precaution.

Subsequent patches for SEV-SNP CPUID validation support will introduce
both such cases. Attempting to disable stack protection for everything
in scope to address that is prohibitive since much of the code, like
SEV-ES #VC handler, is shared code that remains in use after boot and
could benefit from having stack protection enabled. Attempting to inline
calls is brittle and can quickly balloon out to library/helper code
where that's not really an option.

Instead, re-enable stack protection for head32.c/head64.c and make the
appropriate changes to ensure the segment used for the stack canary is
initialized in advance of any stack-protected C calls.

for head64.c:

- The BSP will enter from startup_64 and call into C code
(startup_64_setup_env) shortly after setting up the stack, which may
result in calls to stack-protected code. Set up %gs early to allow
for this safely.
- APs will enter from secondary_startup_64*, and %gs will be set up
soon after. There is one call to C code prior to this
(__startup_secondary_64), but it is only to fetch sme_me_mask, and
unlikely to be stack-protected, so leave things as they are, but add
a note about this in case things change in the future.

for head32.c:

- BSPs/APs will set %fs to __BOOT_DS prior to any C calls. In recent
kernels, the compiler is configured to access the stack canary at
%fs:__stack_chk_guard, which overlaps with the initial per-cpu
__stack_chk_guard variable in the initial/'master' .data..percpu
area. This is sufficient to allow access to the canary for use
during initial startup, so no changes are needed there.

Suggested-by: Joerg Roedel <> #for 64-bit %gs set up
Signed-off-by: Michael Roth <>
Signed-off-by: Brijesh Singh <>
arch/x86/kernel/Makefile | 1 -
arch/x86/kernel/head_64.S | 24 ++++++++++++++++++++++++
2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 2ff3e600f426..4df8c8f7d2ac 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -48,7 +48,6 @@ endif
# non-deterministic coverage.

-CFLAGS_head$(BITS).o += -fno-stack-protector
CFLAGS_cc_platform.o += -fno-stack-protector

CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index d8b3ebd2bb85..7074ebf2b47b 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -65,6 +65,22 @@ SYM_CODE_START_NOALIGN(startup_64)
leaq (__end_init_task - FRAME_SIZE)(%rip), %rsp

leaq _text(%rip), %rdi
+ /*
+ * initial_gs points to initial fixed_per_cpu struct with storage for
+ * the stack protector canary. Global pointer fixups are needed at this
+ * stage, so apply them as is done in fixup_pointer(), and initialize %gs
+ * such that the canary can be accessed at %gs:40 for subsequent C calls.
+ */
+ movl $MSR_GS_BASE, %ecx
+ movq initial_gs(%rip), %rax
+ movq $_text, %rdx
+ subq %rdx, %rax
+ addq %rdi, %rax
+ movq %rax, %rdx
+ shrq $32, %rdx
+ wrmsr
pushq %rsi
call startup_64_setup_env
popq %rsi
@@ -133,6 +149,14 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
* added to the initial pgdir entry that will be programmed into CR3.
pushq %rsi
+ /*
+ * NOTE: %gs at this point is a stale data segment left over from the
+ * real-mode trampoline, so the default stack protector canary location
+ * at %gs:40 does not yet coincide with the expected fixed_per_cpu struct
+ * that contains storage for the stack canary. So take care not to add
+ * anything to the C functions in this path that would result in stack
+ * protected C code being generated.
+ */
call __startup_secondary_64
popq %rsi

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