[PATCH v3 24/39] x86/ibt: Disable IBT around firmware

Peter Zijlstra
2022-03-03T11:33:21Z

Assume firmware isn't IBT clean and disable it across calls.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
arch/x86/include/asm/efi.h | 9 +++++++--
arch/x86/include/asm/ibt.h | 6 ++++++
arch/x86/kernel/apm_32.c | 7 +++++++
arch/x86/kernel/cpu/common.c | 28 ++++++++++++++++++++++++++++
4 files changed, 48 insertions(+), 2 deletions(-)

--- a/arch/x86/include/asm/efi.h
+++ b/arch/x86/include/asm/efi.h
@@ -7,6 +7,7 @@
#include <asm/tlb.h>
#include <asm/nospec-branch.h>
#include <asm/mmu_context.h>
+#include <asm/ibt.h>
#include <linux/build_bug.h>
#include <linux/kernel.h>
#include <linux/pgtable.h>
@@ -120,8 +121,12 @@ extern asmlinkage u64 __efi_call(void *f
efi_enter_mm(); \
})

-#define arch_efi_call_virt(p, f, args...) \
- efi_call((void *)p->f, args) \
+#define arch_efi_call_virt(p, f, args...) ({ \
+ u64 ret, ibt = ibt_save(); \
+ ret = efi_call((void *)p->f, args); \
+ ibt_restore(ibt); \
+ ret; \
+})

#define arch_efi_call_virt_teardown() \
({ \
--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -39,6 +39,9 @@ static inline bool is_endbr(u32 val)
return val == gen_endbr();
}

+extern __noendbr u64 ibt_save(void);
+extern __noendbr void ibt_restore(u64 save);
+
#else /* __ASSEMBLY__ */

#ifdef CONFIG_X86_64
@@ -61,6 +64,9 @@ static inline bool is_endbr(u32 val)

static inline bool is_endbr(u32 val) { return false; }

+static inline u64 ibt_save(void) { return 0; }
+static inline void ibt_restore(u64 save) { }
+
#else /* __ASSEMBLY__ */

#define ENDBR
--- a/arch/x86/kernel/apm_32.c
+++ b/arch/x86/kernel/apm_32.c
@@ -232,6 +232,7 @@
#include <asm/paravirt.h>
#include <asm/reboot.h>
#include <asm/nospec-branch.h>
+#include <asm/ibt.h>

#if defined(CONFIG_APM_DISPLAY_BLANK) && defined(CONFIG_VT)
extern int (*console_blank_hook)(int);
@@ -598,6 +599,7 @@ static long __apm_bios_call(void *_call)
struct desc_struct save_desc_40;
struct desc_struct *gdt;
struct apm_bios_call *call = _call;
+ u64 ibt;

cpu = get_cpu();
BUG_ON(cpu != 0);
@@ -607,11 +609,13 @@ static long __apm_bios_call(void *_call)

apm_irq_save(flags);
firmware_restrict_branch_speculation_start();
+ ibt = ibt_save();
APM_DO_SAVE_SEGS;
apm_bios_call_asm(call->func, call->ebx, call->ecx,
&call->eax, &call->ebx, &call->ecx, &call->edx,
&call->esi);
APM_DO_RESTORE_SEGS;
+ ibt_restore(ibt);
firmware_restrict_branch_speculation_end();
apm_irq_restore(flags);
gdt[0x40 / 8] = save_desc_40;
@@ -676,6 +680,7 @@ static long __apm_bios_call_simple(void
struct desc_struct save_desc_40;
struct desc_struct *gdt;
struct apm_bios_call *call = _call;
+ u64 ibt;

cpu = get_cpu();
BUG_ON(cpu != 0);
@@ -685,10 +690,12 @@ static long __apm_bios_call_simple(void

apm_irq_save(flags);
firmware_restrict_branch_speculation_start();
+ ibt = ibt_save();
APM_DO_SAVE_SEGS;
error = apm_bios_call_simple_asm(call->func, call->ebx, call->ecx,
&call->eax);
APM_DO_RESTORE_SEGS;
+ ibt_restore(ibt);
firmware_restrict_branch_speculation_end();
apm_irq_restore(flags);
gdt[0x40 / 8] = save_desc_40;
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -594,6 +594,34 @@ static __init int setup_disable_pku(char
__setup("nopku", setup_disable_pku);
#endif /* CONFIG_X86_64 */

+#ifdef CONFIG_X86_KERNEL_IBT
+
+__noendbr u64 ibt_save(void)
+{
+ u64 msr = 0;
+
+ if (cpu_feature_enabled(X86_FEATURE_IBT)) {
+ rdmsrl(MSR_IA32_S_CET, msr);
+ wrmsrl(MSR_IA32_S_CET, msr & ~CET_ENDBR_EN);
+ }
+
+ return msr;
+}
+
+__noendbr void ibt_restore(u64 save)
+{
+ u64 msr;
+
+ if (cpu_feature_enabled(X86_FEATURE_IBT)) {
+ rdmsrl(MSR_IA32_S_CET, msr);
+ msr &= ~CET_ENDBR_EN;
+ msr |= (save & CET_ENDBR_EN);
+ wrmsrl(MSR_IA32_S_CET, msr);
+ }
+}
+
+#endif
+
static __always_inline void setup_cet(struct cpuinfo_x86 *c)
{
u64 msr = CET_ENDBR_EN;