x86/entry/32: Enter the kernel via trampoline stack

Use the entry-stack as a trampoline to enter the kernel. The entry-stack is
already in the cpu_entry_area and will be mapped to userspace when PTI is
enabled.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Pavel Machek <pavel@ucw.cz>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: aliguori@amazon.com
Cc: daniel.gruss@iaik.tugraz.at
Cc: hughd@google.com
Cc: keescook@google.com
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Waiman Long <llong@redhat.com>
Cc: "David H . Gutteridge" <dhgutteridge@sympatico.ca>
Cc: joro@8bytes.org
Link: https://lkml.kernel.org/r/1531906876-13451-8-git-send-email-joro@8bytes.org

arch/x86/entry/entry_32.S

@@ -154,7 +154,7 @@
 
 #endif /* CONFIG_X86_32_LAZY_GS */
 
-.macro SAVE_ALL pt_regs_ax=%eax
+.macro SAVE_ALL pt_regs_ax=%eax switch_stacks=0
 	cld
 	PUSH_GS
 	pushl	%fs
@@ -173,6 +173,12 @@
 	movl	$(__KERNEL_PERCPU), %edx
 	movl	%edx, %fs
 	SET_KERNEL_GS %edx
+
+	/* Switch to kernel stack if necessary */
+.if \switch_stacks > 0
+	SWITCH_TO_KERNEL_STACK
+.endif
+
 .endm
 
 /*
@@ -269,6 +275,73 @@
 .Lend_\@:
 #endif /* CONFIG_X86_ESPFIX32 */
 .endm
+
+
+/*
+ * Called with pt_regs fully populated and kernel segments loaded,
+ * so we can access PER_CPU and use the integer registers.
+ *
+ * We need to be very careful here with the %esp switch, because an NMI
+ * can happen everywhere. If the NMI handler finds itself on the
+ * entry-stack, it will overwrite the task-stack and everything we
+ * copied there. So allocate the stack-frame on the task-stack and
+ * switch to it before we do any copying.
+ */
+.macro SWITCH_TO_KERNEL_STACK
+
+	ALTERNATIVE     "", "jmp .Lend_\@", X86_FEATURE_XENPV
+
+	/* Are we on the entry stack? Bail out if not! */
+	movl	PER_CPU_VAR(cpu_entry_area), %ecx
+	addl	$CPU_ENTRY_AREA_entry_stack + SIZEOF_entry_stack, %ecx
+	subl	%esp, %ecx	/* ecx = (end of entry_stack) - esp */
+	cmpl	$SIZEOF_entry_stack, %ecx
+	jae	.Lend_\@
+
+	/* Load stack pointer into %esi and %edi */
+	movl	%esp, %esi
+	movl	%esi, %edi
+
+	/* Move %edi to the top of the entry stack */
+	andl	$(MASK_entry_stack), %edi
+	addl	$(SIZEOF_entry_stack), %edi
+
+	/* Load top of task-stack into %edi */
+	movl	TSS_entry2task_stack(%edi), %edi
+
+	/* Bytes to copy */
+	movl	$PTREGS_SIZE, %ecx
+
+#ifdef CONFIG_VM86
+	testl	$X86_EFLAGS_VM, PT_EFLAGS(%esi)
+	jz	.Lcopy_pt_regs_\@
+
+	/*
+	 * Stack-frame contains 4 additional segment registers when
+	 * coming from VM86 mode
+	 */
+	addl	$(4 * 4), %ecx
+
+.Lcopy_pt_regs_\@:
+#endif
+
+	/* Allocate frame on task-stack */
+	subl	%ecx, %edi
+
+	/* Switch to task-stack */
+	movl	%edi, %esp
+
+	/*
+	 * We are now on the task-stack and can safely copy over the
+	 * stack-frame
+	 */
+	shrl	$2, %ecx
+	cld
+	rep movsl
+
+.Lend_\@:
+.endm
+
 /*
  * %eax: prev task
  * %edx: next task
@@ -469,7 +542,7 @@ ENTRY(entry_SYSENTER_32)
 	pushl	$__USER_CS		/* pt_regs->cs */
 	pushl	$0			/* pt_regs->ip = 0 (placeholder) */
 	pushl	%eax			/* pt_regs->orig_ax */
-	SAVE_ALL pt_regs_ax=$-ENOSYS	/* save rest */
+	SAVE_ALL pt_regs_ax=$-ENOSYS	/* save rest, stack already switched */
 
 	/*
 	 * SYSENTER doesn't filter flags, so we need to clear NT, AC
@@ -580,7 +653,8 @@ ENDPROC(entry_SYSENTER_32)
 ENTRY(entry_INT80_32)
 	ASM_CLAC
 	pushl	%eax			/* pt_regs->orig_ax */
-	SAVE_ALL pt_regs_ax=$-ENOSYS	/* save rest */
+
+	SAVE_ALL pt_regs_ax=$-ENOSYS switch_stacks=1	/* save rest */
 
 	/*
 	 * User mode is traced as though IRQs are on, and the interrupt gate
@@ -677,7 +751,8 @@ END(irq_entries_start)
 common_interrupt:
 	ASM_CLAC
 	addl	$-0x80, (%esp)			/* Adjust vector into the [-256, -1] range */
-	SAVE_ALL
+
+	SAVE_ALL switch_stacks=1
 	ENCODE_FRAME_POINTER
 	TRACE_IRQS_OFF
 	movl	%esp, %eax
@@ -685,16 +760,16 @@ common_interrupt:
 	jmp	ret_from_intr
 ENDPROC(common_interrupt)
 
-#define BUILD_INTERRUPT3(name, nr, fn)	\
-ENTRY(name)				\
-	ASM_CLAC;			\
-	pushl	$~(nr);			\
-	SAVE_ALL;			\
-	ENCODE_FRAME_POINTER;		\
-	TRACE_IRQS_OFF			\
-	movl	%esp, %eax;		\
-	call	fn;			\
-	jmp	ret_from_intr;		\
+#define BUILD_INTERRUPT3(name, nr, fn)			\
+ENTRY(name)						\
+	ASM_CLAC;					\
+	pushl	$~(nr);					\
+	SAVE_ALL switch_stacks=1;			\
+	ENCODE_FRAME_POINTER;				\
+	TRACE_IRQS_OFF					\
+	movl	%esp, %eax;				\
+	call	fn;					\
+	jmp	ret_from_intr;				\
 ENDPROC(name)
 
 #define BUILD_INTERRUPT(name, nr)		\
@@ -926,16 +1001,20 @@ common_exception:
 	pushl	%es
 	pushl	%ds
 	pushl	%eax
+	movl	$(__USER_DS), %eax
+	movl	%eax, %ds
+	movl	%eax, %es
+	movl	$(__KERNEL_PERCPU), %eax
+	movl	%eax, %fs
 	pushl	%ebp
 	pushl	%edi
 	pushl	%esi
 	pushl	%edx
 	pushl	%ecx
 	pushl	%ebx
+	SWITCH_TO_KERNEL_STACK
 	ENCODE_FRAME_POINTER
 	cld
-	movl	$(__KERNEL_PERCPU), %ecx
-	movl	%ecx, %fs
 	UNWIND_ESPFIX_STACK
 	GS_TO_REG %ecx
 	movl	PT_GS(%esp), %edi		# get the function address
@@ -943,9 +1022,6 @@ common_exception:
 	movl	$-1, PT_ORIG_EAX(%esp)		# no syscall to restart
 	REG_TO_PTGS %ecx
 	SET_KERNEL_GS %ecx
-	movl	$(__USER_DS), %ecx
-	movl	%ecx, %ds
-	movl	%ecx, %es
 	TRACE_IRQS_OFF
 	movl	%esp, %eax			# pt_regs pointer
 	CALL_NOSPEC %edi
@@ -964,6 +1040,7 @@ ENTRY(debug)
 	 */
 	ASM_CLAC
 	pushl	$-1				# mark this as an int
+
 	SAVE_ALL
 	ENCODE_FRAME_POINTER
 	xorl	%edx, %edx			# error code 0
@@ -999,6 +1076,7 @@ END(debug)
  */
 ENTRY(nmi)
 	ASM_CLAC
+
 #ifdef CONFIG_X86_ESPFIX32
 	pushl	%eax
 	movl	%ss, %eax
@@ -1066,7 +1144,8 @@ END(nmi)
 ENTRY(int3)
 	ASM_CLAC
 	pushl	$-1				# mark this as an int
-	SAVE_ALL
+
+	SAVE_ALL switch_stacks=1
 	ENCODE_FRAME_POINTER
 	TRACE_IRQS_OFF
 	xorl	%edx, %edx			# zero error code

arch/x86/include/asm/switch_to.h

@@ -89,13 +89,23 @@ static inline void refresh_sysenter_cs(struct thread_struct *thread)
 /* This is used when switching tasks or entering/exiting vm86 mode. */
 static inline void update_sp0(struct task_struct *task)
 {
-	/* On x86_64, sp0 always points to the entry trampoline stack, which is constant: */
+	/* sp0 always points to the entry trampoline stack, which is constant: */
 #ifdef CONFIG_X86_32
-	load_sp0(task->thread.sp0);
+	if (static_cpu_has(X86_FEATURE_XENPV))
+		load_sp0(task->thread.sp0);
+	else
+		this_cpu_write(cpu_tss_rw.x86_tss.sp1, task->thread.sp0);
 #else
+	/*
+	 * x86-64 updates x86_tss.sp1 via cpu_current_top_of_stack. That
+	 * doesn't work on x86-32 because sp1 and
+	 * cpu_current_top_of_stack have different values (because of
+	 * the non-zero stack-padding on 32bit).
+	 */
 	if (static_cpu_has(X86_FEATURE_XENPV))
 		load_sp0(task_top_of_stack(task));
 #endif
+
 }
 
 #endif /* _ASM_X86_SWITCH_TO_H */

arch/x86/kernel/asm-offsets.c

@@ -103,6 +103,7 @@ void common(void) {
 	OFFSET(CPU_ENTRY_AREA_entry_trampoline, cpu_entry_area, entry_trampoline);
 	OFFSET(CPU_ENTRY_AREA_entry_stack, cpu_entry_area, entry_stack_page);
 	DEFINE(SIZEOF_entry_stack, sizeof(struct entry_stack));
+	DEFINE(MASK_entry_stack, (~(sizeof(struct entry_stack) - 1)));
 
 	/* Offset for sp0 and sp1 into the tss_struct */
 	OFFSET(TSS_sp0, tss_struct, x86_tss.sp0);

arch/x86/kernel/cpu/common.c

@@ -1804,11 +1804,12 @@ void cpu_init(void)
 	enter_lazy_tlb(&init_mm, curr);
 
 	/*
-	 * Initialize the TSS.  Don't bother initializing sp0, as the initial
-	 * task never enters user mode.
+	 * Initialize the TSS.  sp0 points to the entry trampoline stack
+	 * regardless of what task is running.
 	 */
 	set_tss_desc(cpu, &get_cpu_entry_area(cpu)->tss.x86_tss);
 	load_TR_desc();
+	load_sp0((unsigned long)(cpu_entry_stack(cpu) + 1));
 
 	load_mm_ldt(&init_mm);
 

arch/x86/kernel/process.c

@@ -57,14 +57,12 @@ __visible DEFINE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss_rw) = {
 		 */
 		.sp0 = (1UL << (BITS_PER_LONG-1)) + 1,
 
-#ifdef CONFIG_X86_64
 		/*
 		 * .sp1 is cpu_current_top_of_stack.  The init task never
 		 * runs user code, but cpu_current_top_of_stack should still
 		 * be well defined before the first context switch.
 		 */
 		.sp1 = TOP_OF_INIT_STACK,
-#endif
 
 #ifdef CONFIG_X86_32
 		.ss0 = __KERNEL_DS,

arch/x86/kernel/process_32.c

@@ -290,8 +290,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	this_cpu_write(cpu_current_top_of_stack,
 		       (unsigned long)task_stack_page(next_p) +
 		       THREAD_SIZE);
-	/* SYSENTER reads the task-stack from tss.sp1 */
-	this_cpu_write(cpu_tss_rw.x86_tss.sp1, next_p->thread.sp0);
 
 	/*
 	 * Restore %gs if needed (which is common)