[RFC][PATCH 15/24] x86/pti: Execute syscall functions on the kernel stack

[Alexandre Chartre <alexandre.chartre@oracle.com>]

During a syscall, the kernel is entered and it switches the stack
to the PTI stack which is mapped both in the kernel and in the
user page-table. When executing the syscall function, switch to
the kernel stack (which is mapped only in the kernel page-table)
so that no kernel data leak to the userland through the stack.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
---
 arch/x86/entry/common.c          | 11 ++++++++++-
 arch/x86/entry/entry_64.S        |  1 +
 arch/x86/include/asm/irq_stack.h |  3 +++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 54d0931801e1..ead6a4c72e6a 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -56,10 +56,19 @@ __visible noinstr void return_from_fork(struct pt_regs *regs,
 static __always_inline void run_syscall(sys_call_ptr_t sysfunc,
 					struct pt_regs *regs)
 {
+	unsigned long stack;
+
 	if (!sysfunc)
 		return;
 
-	regs->ax = sysfunc(regs);
+	if (!pti_enabled()) {
+		regs->ax = sysfunc(regs);
+		return;
+	}
+
+	stack = (unsigned long)task_top_of_kernel_stack(current);
+	regs->ax = asm_call_syscall_on_stack((void *)(stack - 8),
+					     sysfunc, regs);
 }
 
 #ifdef CONFIG_X86_64
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 29beab46bedd..6b88a0eb8975 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -771,6 +771,7 @@ SYM_FUNC_START(asm_call_on_stack_2)
 SYM_FUNC_START(asm_call_on_stack_3)
 SYM_INNER_LABEL(asm_call_sysvec_on_stack, SYM_L_GLOBAL)
 SYM_INNER_LABEL(asm_call_irq_on_stack, SYM_L_GLOBAL)
+SYM_INNER_LABEL(asm_call_syscall_on_stack, SYM_L_GLOBAL)
 	/*
 	 * Save the frame pointer unconditionally. This allows the ORC
 	 * unwinder to handle the stack switch.
diff --git a/arch/x86/include/asm/irq_stack.h b/arch/x86/include/asm/irq_stack.h
index 359427216336..108d9da7c01c 100644
--- a/arch/x86/include/asm/irq_stack.h
+++ b/arch/x86/include/asm/irq_stack.h
@@ -5,6 +5,7 @@
 #include <linux/ptrace.h>
 
 #include <asm/processor.h>
+#include <asm/syscall.h>
 
 #ifdef CONFIG_X86_64
 static __always_inline bool irqstack_active(void)
@@ -25,6 +26,8 @@ void asm_call_sysvec_on_stack(void *sp, void (*func)(struct pt_regs *regs),
 			      struct pt_regs *regs);
 void asm_call_irq_on_stack(void *sp, void (*func)(struct irq_desc *desc),
 			   struct irq_desc *desc);
+long asm_call_syscall_on_stack(void *sp, sys_call_ptr_t func,
+			       struct pt_regs *regs);
 
 static __always_inline void __run_on_irqstack(void (*func)(void))
 {
-- 
2.18.4