x86 setup: guard the heap against invalid stack setups

If we use the bootloader-provided stack pointer, we might end up in a
situation where the bootloader (incorrectly) pointed the stack in the
middle of our heap.  Catch this by simply comparing the computed heap
end value to the stack pointer minus the defined stack size.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/boot/main.c

@@ -100,20 +100,32 @@ static void set_bios_mode(void)
 #endif
 }
 
-void main(void)
+static void init_heap(void)
 {
-	/* First, copy the boot header into the "zeropage" */
-	copy_boot_params();
+	char *stack_end;
 
-	/* End of heap check */
 	if (boot_params.hdr.loadflags & CAN_USE_HEAP) {
-		heap_end = (char *)(boot_params.hdr.heap_end_ptr
-				    +0x200-STACK_SIZE);
+		asm("leal %P1(%%esp),%0"
+		    : "=r" (stack_end) : "i" (-STACK_SIZE));
+
+		heap_end = (char *)
+			((size_t)boot_params.hdr.heap_end_ptr + 0x200);
+		if (heap_end > stack_end)
+			heap_end = stack_end;
 	} else {
 		/* Boot protocol 2.00 only, no heap available */
 		puts("WARNING: Ancient bootloader, some functionality "
 		     "may be limited!\n");
 	}
+}
+
+void main(void)
+{
+	/* First, copy the boot header into the "zeropage" */
+	copy_boot_params();
+
+	/* End of heap check */
+	init_heap();
 
 	/* Make sure we have all the proper CPU support */
 	if (validate_cpu()) {