um: allocate a guard page to helper threads

We've been running into stack overflows in helper threads
corrupting memory (e.g. because somebody put printf() or
os_info() there), so to avoid those causing hard-to-debug
issues later on, allocate a guard page for helper thread
stacks and mark it read-only.

Unfortunately, the crash dump at that point is useless as
the stack tracer will try to backtrace the *kernel* thread,
not the helper thread, but at least we don't survive to a
random issue caused by corruption.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/ubd_kern.c

@@ -1241,7 +1241,7 @@ static int __init ubd_driver_init(void){
 		/* Letting ubd=sync be like using ubd#s= instead of ubd#= is
 		 * enough. So use anyway the io thread. */
 	}
-	stack = alloc_stack(0, 0);
+	stack = alloc_stack(0);
 	io_pid = start_io_thread(stack + PAGE_SIZE - sizeof(void *),
 				 &thread_fd);
 	if(io_pid < 0){

arch/um/include/shared/kern_util.h

@@ -19,7 +19,7 @@ extern int kmalloc_ok;
 #define UML_ROUND_UP(addr) \
 	((((unsigned long) addr) + PAGE_SIZE - 1) & PAGE_MASK)
 
-extern unsigned long alloc_stack(int order, int atomic);
+extern unsigned long alloc_stack(int atomic);
 extern void free_stack(unsigned long stack, int order);
 
 struct pt_regs;

arch/um/kernel/process.c

@@ -32,6 +32,7 @@
 #include <os.h>
 #include <skas.h>
 #include <linux/time-internal.h>
+#include <asm/set_memory.h>
 
 /*
  * This is a per-cpu array.  A processor only modifies its entry and it only
@@ -62,16 +63,18 @@ void free_stack(unsigned long stack, int order)
 	free_pages(stack, order);
 }
 
-unsigned long alloc_stack(int order, int atomic)
+unsigned long alloc_stack(int atomic)
 {
-	unsigned long page;
+	unsigned long addr;
 	gfp_t flags = GFP_KERNEL;
 
 	if (atomic)
 		flags = GFP_ATOMIC;
-	page = __get_free_pages(flags, order);
+	addr = __get_free_pages(flags, 1);
 
-	return page;
+	set_memory_ro(addr, 1);
+
+	return addr + PAGE_SIZE;
 }
 
 static inline void set_current(struct task_struct *task)

arch/um/os-Linux/helper.c

@@ -45,7 +45,7 @@ int run_helper(void (*pre_exec)(void *), void *pre_data, char **argv)
 	unsigned long stack, sp;
 	int pid, fds[2], ret, n;
 
-	stack = alloc_stack(0, __cant_sleep());
+	stack = alloc_stack(__cant_sleep());
 	if (stack == 0)
 		return -ENOMEM;
 
@@ -116,7 +116,7 @@ int run_helper_thread(int (*proc)(void *), void *arg, unsigned int flags,
 	unsigned long stack, sp;
 	int pid, status, err;
 
-	stack = alloc_stack(0, __cant_sleep());
+	stack = alloc_stack(__cant_sleep());
 	if (stack == 0)
 		return -ENOMEM;