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>

1 files changed, 7 insertions, 4 deletions

diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index 81d508daf67c..2a986ece5478 100644
--- a/arch/um/kernel/process.c
+++ b/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)