[PATCH v9 21/43] x86/head/64: Re-enable stack protection

Brijesh Singh
2022-01-28T17:18:56Z

From: Michael Roth <michael.roth@amd.com>

Due to the following commit:

103a4908ad4d ("x86/head/64: Disable stack protection for head$(BITS).o")

kernel/head{32,64}.c are 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.

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 %gs being setup
(__startup_secondary_64()), but it is only to fetch 'sme_me_mask'
global, so just load 'sme_me_mask' directly instead, and remove the
now-unused __startup_secondary_64() function.

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 [1], 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.

[1] 3fb0fdb3bbe7 ("x86/stackprotector/32: Make the canary into a regular percpu variable")

Suggested-by: Joerg Roedel <jroedel@suse.de> #for 64-bit %gs set up
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
arch/x86/include/asm/setup.h | 1 -
arch/x86/kernel/Makefile | 1 -
arch/x86/kernel/head64.c | 9 ---------
arch/x86/kernel/head_64.S | 24 +++++++++++++++++++++---
4 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index a12458a7a8d4..72ede9159951 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -49,7 +49,6 @@ extern unsigned long saved_video_mode;
extern void reserve_standard_io_resources(void);
extern void i386_reserve_resources(void);
extern unsigned long __startup_64(unsigned long physaddr, struct boot_params *bp);
-extern unsigned long __startup_secondary_64(void);
extern void startup_64_setup_env(unsigned long physbase);
extern void early_setup_idt(void);
extern void __init do_early_exception(struct pt_regs *regs, int trapnr);
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 6aef9ee28a39..bd45e5ee6fe3 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/head64.c b/arch/x86/kernel/head64.c
index 1239bc104cda..c80952dded32 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -319,15 +319,6 @@ unsigned long __head __startup_64(unsigned long physaddr,
return sme_postprocess_startup(bp, pmd);
}

-unsigned long __startup_secondary_64(void)
-{
- /*
- * Return the SME encryption mask (if SME is active) to be used as a
- * modifier for the initial pgdir entry programmed into CR3.
- */
- return sme_get_me_mask();
-}
-
/* Wipe all early page tables except for the kernel symbol map */
static void __init reset_early_page_tables(void)
{
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 9c2c3aff5ee4..9e84263bcb94 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_percpu_data 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
@@ -145,9 +161,11 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
* Retrieve the modifier (SME encryption mask if SME is active) to be
* added to the initial pgdir entry that will be programmed into CR3.
*/
- pushq %rsi
- call __startup_secondary_64
- popq %rsi
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+ movq sme_me_mask, %rax
+#else
+ xorq %rax, %rax
+#endif

/* Form the CR3 value being sure to include the CR3 modifier */
addq $(init_top_pgt - __START_KERNEL_map), %rax
--
2.25.1