linux-kernel - [PATCH 06/15] x86: Avoid CFI jump tables in IDT and entry points

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Message-Id: <20210416203844.3803177-7-samitolvanen@google.com>
Date:   Fri, 16 Apr 2021 13:38:35 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     x86@...nel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH 06/15] x86: Avoid CFI jump tables in IDT and entry points

With CONFIG_CFI_CLANG, the compiler replaces function addresses in C
code with jump table addresses. To avoid referring to jump tables in
entry code with PTI, disable CFI for IDT and paravirt code, and use
function_nocfi() to prevent jump table addresses from being added to
the IDT or system call entry points.

Reported-by: Sedat Dilek <sedat.dilek@...il.com>
Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>
---
 arch/x86/include/asm/desc.h  | 8 +++++++-
 arch/x86/kernel/Makefile     | 3 +++
 arch/x86/kernel/cpu/common.c | 8 ++++----
 arch/x86/kernel/idt.c        | 2 +-
 arch/x86/kernel/traps.c      | 2 +-
 arch/x86/xen/Makefile        | 2 ++
 6 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 476082a83d1c..96666256eec2 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -381,7 +381,13 @@ static inline void set_desc_limit(struct desc_struct *desc, unsigned long limit)
 	desc->limit1 = (limit >> 16) & 0xf;
 }
 
-void alloc_intr_gate(unsigned int n, const void *addr);
+/*
+ * Use function_nocfi() to prevent the compiler from replacing function
+ * addresses with jump table addresses when CONFIG_CFI_CLANG is enabled.
+ */
+#define alloc_intr_gate(n, addr) __alloc_intr_gate(n, function_nocfi(addr))
+
+void __alloc_intr_gate(unsigned int n, const void *addr);
 
 static inline void init_idt_data(struct idt_data *data, unsigned int n,
 				 const void *addr)
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 0704c2a94272..12a739eb208e 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -46,6 +46,9 @@ endif
 # non-deterministic coverage.
 KCOV_INSTRUMENT		:= n
 
+CFLAGS_REMOVE_idt.o		:= $(CC_FLAGS_CFI)
+CFLAGS_REMOVE_paravirt.o	:= $(CC_FLAGS_CFI)
+
 CFLAGS_head$(BITS).o	+= -fno-stack-protector
 
 CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 6bdb69a9a7dc..e6cff98b182a 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1752,10 +1752,10 @@ DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) = TOP_OF_INIT_STACK;
 void syscall_init(void)
 {
 	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
-	wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
+	wrmsrl(MSR_LSTAR, (unsigned long)function_nocfi(entry_SYSCALL_64));
 
 #ifdef CONFIG_IA32_EMULATION
-	wrmsrl(MSR_CSTAR, (unsigned long)entry_SYSCALL_compat);
+	wrmsrl(MSR_CSTAR, (unsigned long)function_nocfi(entry_SYSCALL_compat));
 	/*
 	 * This only works on Intel CPUs.
 	 * On AMD CPUs these MSRs are 32-bit, CPU truncates MSR_IA32_SYSENTER_EIP.
@@ -1765,9 +1765,9 @@ void syscall_init(void)
 	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS);
 	wrmsrl_safe(MSR_IA32_SYSENTER_ESP,
 		    (unsigned long)(cpu_entry_stack(smp_processor_id()) + 1));
-	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)entry_SYSENTER_compat);
+	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)function_nocfi(entry_SYSENTER_compat));
 #else
-	wrmsrl(MSR_CSTAR, (unsigned long)ignore_sysret);
+	wrmsrl(MSR_CSTAR, (unsigned long)function_nocfi(ignore_sysret));
 	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)GDT_ENTRY_INVALID_SEG);
 	wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL);
 	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, 0ULL);
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index d552f177eca0..6574a742e373 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -340,7 +340,7 @@ void idt_invalidate(void *addr)
 	load_idt(&idt);
 }
 
-void __init alloc_intr_gate(unsigned int n, const void *addr)
+void __init __alloc_intr_gate(unsigned int n, const void *addr)
 {
 	if (WARN_ON(n < FIRST_SYSTEM_VECTOR))
 		return;
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 853ea7a80806..54616dc49116 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -403,7 +403,7 @@ DEFINE_IDTENTRY_DF(exc_double_fault)
 		 * which is what the stub expects, given that the faulting
 		 * RIP will be the IRET instruction.
 		 */
-		regs->ip = (unsigned long)asm_exc_general_protection;
+		regs->ip = (unsigned long)function_nocfi(asm_exc_general_protection);
 		regs->sp = (unsigned long)&gpregs->orig_ax;
 
 		return;
diff --git a/arch/x86/xen/Makefile b/arch/x86/xen/Makefile
index 40b5779fce21..61e2d9b2fa43 100644
--- a/arch/x86/xen/Makefile
+++ b/arch/x86/xen/Makefile
@@ -11,6 +11,8 @@ endif
 CFLAGS_enlighten_pv.o		:= -fno-stack-protector
 CFLAGS_mmu_pv.o			:= -fno-stack-protector
 
+CFLAGS_REMOVE_enlighten_pv.o	:= $(CC_FLAGS_CFI)
+
 obj-y				+= enlighten.o
 obj-y				+= mmu.o
 obj-y				+= time.o
-- 
2.31.1.368.gbe11c130af-goog