From: Michael Roth <redacted>

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 <redacted> #for 64-bit %gs set up
Signed-off-by: Michael Roth <redacted>
Signed-off-by: Brijesh Singh <redacted>
---
 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.
 KCOV_INSTRUMENT		:= n
 
-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
 

-- 
2.25.1