[RFC][PATCH 21/24] x86/entry: Disable stack-protector for IST entry C handlers

[Alexandre Chartre]

The stack-protector option adds a stack canary to functions vulnerable
to stack buffer overflow. The stack canary is defined through the GS
register. Add an attribute to disable the stack-protector option; it
will be used for C functions which can be called while the GS register
might not be properly configured yet.

The GS register is not properly configured for the kernel when we enter
the kernel from userspace. The assembly entry code sets the GS register
for the kernel using the swapgs instruction or the paranoid_entry function,
and so, currently, the GS register is correctly configured when assembly
entry code subsequently transfer control to C code.

Deferring the CR3 register switch from assembly to C code will require to
reimplement paranoid_entry in C and hence also defer the GS register setup
for IST entries to C code. To prepare this change, disable stack-protector
for IST entry C handlers where the GS register setup will eventually
happen.

Signed-off-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
---
 arch/x86/include/asm/idtentry.h | 25 ++++++++++++++++++++-----
 arch/x86/kernel/nmi.c           |  2 +-
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index a6725afaaec0..647af7ea3bf1 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -94,6 +94,21 @@ void run_sysvec(void (*func)(struct pt_regs *regs), struct pt_regs *regs)
 		run_sysvec_on_irqstack_cond(func, regs);
 }
 
+/*
+ * Attribute to disable the stack-protector option. The option is
+ * disabled using the optimize attribute which clears all optimize
+ * options. So we need to specify the optimize option to disable but
+ * also optimize options we want to preserve.
+ *
+ * The stack-protector option adds a stack canary to functions
+ * vulnerable to stack buffer overflow. The stack canary is defined
+ * through the GS register. So the attribute is used to disable the
+ * stack-protector option for functions which can be called while the
+ * GS register might not be properly configured yet.
+ */
+#define no_stack_protector	\
+	__attribute__ ((optimize("-O2,-fno-stack-protector,-fno-omit-frame-pointer")))
+
 /**
  * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
  *		      No error code pushed by hardware
@@ -410,7 +425,7 @@ static __always_inline void __##func(struct pt_regs *regs)
  * Maps to DEFINE_IDTENTRY_RAW
  */
 #define DEFINE_IDTENTRY_IST(func)					\
-	DEFINE_IDTENTRY_RAW(func)
+	no_stack_protector DEFINE_IDTENTRY_RAW(func)
 
 /**
  * DEFINE_IDTENTRY_NOIST - Emit code for NOIST entry points which
@@ -440,7 +455,7 @@ static __always_inline void __##func(struct pt_regs *regs)
  * Maps to DEFINE_IDTENTRY_RAW_ERRORCODE
  */
 #define DEFINE_IDTENTRY_DF(func)					\
-	DEFINE_IDTENTRY_RAW_ERRORCODE(func)
+	no_stack_protector DEFINE_IDTENTRY_RAW_ERRORCODE(func)
 
 /**
  * DEFINE_IDTENTRY_VC_SAFE_STACK - Emit code for VMM communication handler
@@ -472,7 +487,7 @@ static __always_inline void __##func(struct pt_regs *regs)
  * VMM communication handler.
  */
 #define DEFINE_IDTENTRY_VC_SETUP_STACK(func)			\
-	__visible noinstr					\
+	no_stack_protector __visible noinstr			\
 	unsigned long setup_stack_##func(struct pt_regs *regs)
 
 /**
@@ -482,7 +497,7 @@ static __always_inline void __##func(struct pt_regs *regs)
  * Maps to DEFINE_IDTENTRY_RAW_ERRORCODE
  */
 #define DEFINE_IDTENTRY_VC(func)					\
-	DEFINE_IDTENTRY_RAW_ERRORCODE(func)
+	no_stack_protector DEFINE_IDTENTRY_RAW_ERRORCODE(func)
 
 #else	/* CONFIG_X86_64 */
 
@@ -517,7 +532,7 @@ __visible noinstr void func(struct pt_regs *regs,			\
 
 /* C-Code mapping */
 #define DECLARE_IDTENTRY_NMI		DECLARE_IDTENTRY_RAW
-#define DEFINE_IDTENTRY_NMI		DEFINE_IDTENTRY_RAW
+#define DEFINE_IDTENTRY_NMI		no_stack_protector DEFINE_IDTENTRY_RAW
 
 #ifdef CONFIG_X86_64
 #define DECLARE_IDTENTRY_MCE		DECLARE_IDTENTRY_IST
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index be0f654c3095..b6291b683be1 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -473,7 +473,7 @@ static DEFINE_PER_CPU(enum nmi_states, nmi_state);
 static DEFINE_PER_CPU(unsigned long, nmi_cr2);
 static DEFINE_PER_CPU(unsigned long, nmi_dr7);
 
-DEFINE_IDTENTRY_RAW(exc_nmi)
+DEFINE_IDTENTRY_NMI(exc_nmi)
 {
 	bool irq_state;
 
-- 
2.18.4