x86-32: Make sure the stack is set up before we use it

Since checkin ebba638ae723d8a8fc2f7abce5ec18b688b791d7 we call
verify_cpu even in 32-bit mode. Unfortunately, calling a function
means using the stack, and the stack pointer was not initialized in
the 32-bit setup code! This code initializes the stack pointer, and
simplifies the interface slightly since it is easier to rely on just a
pointer value rather than a descriptor; we need to have different
values for the segment register anyway.

This retains start_stack as a virtual address, even though a physical
address would be more convenient for 32 bits; the 64-bit code wants
the other way around...

Reported-by: Matthieu Castet <[e-mail]>
LKML-Reference: <[e-mail]>
Tested-by: Kees Cook <[e-mail]>
Signed-off-by: H. Peter Anvin <[e-mail]>

arch/x86/include/asm/smp.h

···
 DECLARE_EARLY_PER_CPU(u16, x86_bios_cpu_apicid);
 
 /* Static state in head.S used to set up a CPU */
-extern struct {
-	void *sp;
-	unsigned short ss;
-} stack_start;
+extern unsigned long stack_start; /* Initial stack pointer address */
 
 struct smp_ops {
 	void (*smp_prepare_boot_cpu)(void);

arch/x86/kernel/acpi/sleep.c

···
 #else /* CONFIG_64BIT */
 	header->trampoline_segment = setup_trampoline() >> 4;
 #ifdef CONFIG_SMP
-	stack_start.sp = temp_stack + sizeof(temp_stack);
+	stack_start = (unsigned long)temp_stack + sizeof(temp_stack);
 	early_gdt_descr.address =
 			(unsigned long)get_cpu_gdt_table(smp_processor_id());
 	initial_gs = per_cpu_offset(smp_processor_id());

arch/x86/kernel/head_32.S

···
  */
 __HEAD
 ENTRY(startup_32)
+	movl pa(stack_start),%ecx
+	
 	/* test KEEP_SEGMENTS flag to see if the bootloader is asking
 		us to not reload segments */
 	testb $(1<<6), BP_loadflags(%esi)
···
 	movl %eax,%es
 	movl %eax,%fs
 	movl %eax,%gs
+	movl %eax,%ss
 2:
+	leal -__PAGE_OFFSET(%ecx),%esp
 
 /*
  * Clear BSS first so that there are no surprises...
···
  * _brk_end is set up to point to the first "safe" location.
  * Mappings are created both at virtual address 0 (identity mapping)
  * and PAGE_OFFSET for up to _end.
- *
- * Note that the stack is not yet set up!
  */
 #ifdef CONFIG_X86_PAE
 
···
 	movl %eax,%es
 	movl %eax,%fs
 	movl %eax,%gs
+	movl pa(stack_start),%ecx
+	movl %eax,%ss
+	leal -__PAGE_OFFSET(%ecx),%esp
 #endif /* CONFIG_SMP */
 default_entry:
 
···
 	movl %eax,%cr0		/* ..and set paging (PG) bit */
 	ljmp $__BOOT_CS,$1f	/* Clear prefetch and normalize %eip */
 1:
-	/* Set up the stack pointer */
-	lss stack_start,%esp
+	/* Shift the stack pointer to a virtual address */
+	addl $__PAGE_OFFSET, %esp
 
 /*
  * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
···
 
 #ifdef CONFIG_SMP
 	cmpb $0, ready
-	jz  1f				/* Initial CPU cleans BSS */
-	jmp checkCPUtype
-1:
+	jnz checkCPUtype
 #endif /* CONFIG_SMP */
 
 /*
···
 
 	cld			# gcc2 wants the direction flag cleared at all times
 	pushl $0		# fake return address for unwinder
-#ifdef CONFIG_SMP
-	movb ready, %cl
 	movb $1, ready
-	cmpb $0,%cl		# the first CPU calls start_kernel
-	je   1f
-	movl (stack_start), %esp
-1:
-#endif /* CONFIG_SMP */
 	jmp *(initial_code)
 
 /*
···
 #endif
 
 .data
+.balign 4
 ENTRY(stack_start)
 	.long init_thread_union+THREAD_SIZE
-	.long __BOOT_DS
-
-ready:	.byte 0
 
 early_recursion_flag:
 	.long 0
+
+ready:	.byte 0
 
 int_msg:
 	.asciz "Unknown interrupt or fault at: %p %p %p\n"

arch/x86/kernel/smpboot.c

···
 	 * target processor state.
 	 */
 	startup_ipi_hook(phys_apicid, (unsigned long) start_secondary,
-			 (unsigned long)stack_start.sp);
+			 stack_start);
 
 	/*
 	 * Run STARTUP IPI loop.
···
 #endif
 	early_gdt_descr.address = (unsigned long)get_cpu_gdt_table(cpu);
 	initial_code = (unsigned long)start_secondary;
-	stack_start.sp = (void *) c_idle.idle->thread.sp;
+	stack_start  = c_idle.idle->thread.sp;
 
 	/* start_ip had better be page-aligned! */
 	start_ip = setup_trampoline();