[PATCH] Restrict stack space reservation to rlimit

[Michael Neuling <mikey@neuling.org>]

When reserving stack space for a new process, make sure we're not
attempting to allocate more than rlimit allows.

Also, reserve the same stack size independent of page size.

This fixes a bug cause by b6a2fea39318e43fee84fa7b0b90d68bed92d2ba 
"mm: variable length argument support" and unmasked by
fc63cf237078c86214abcb2ee9926d8ad289da9b 
"exec: setup_arg_pages() fails to return errors".

Signed-off-by: Michael Neuling <mikey@neuling.org>
Cc: Anton Blanchard <anton@samba.org>
Cc: stable@kernel.org
---
Update commit message to include patch name and SHA1 of related
patches.  

 fs/exec.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Index: clone1/fs/exec.c
===================================================================
--- clone1.orig/fs/exec.c
+++ clone1/fs/exec.c
@@ -554,7 +554,7 @@ static int shift_arg_pages(struct vm_are
 	return 0;
 }
 
-#define EXTRA_STACK_VM_PAGES	20	/* random */
+#define EXTRA_STACK_VM_SIZE	81920UL	/* randomly 20 4K pages */
 
 /*
  * Finalizes the stack vm_area_struct. The flags and permissions are updated,
@@ -627,10 +627,13 @@ int setup_arg_pages(struct linux_binprm 
 			goto out_unlock;
 	}
 
+	stack_base = min(EXTRA_STACK_VM_SIZE,
+			 current->signal->rlim[RLIMIT_STACK].rlim_cur) -
+		PAGE_SIZE;
 #ifdef CONFIG_STACK_GROWSUP
-	stack_base = vma->vm_end + EXTRA_STACK_VM_PAGES * PAGE_SIZE;
+	stack_base = vma->vm_end + stack_base;
 #else
-	stack_base = vma->vm_start - EXTRA_STACK_VM_PAGES * PAGE_SIZE;
+	stack_base = vma->vm_start - stack_base;
 #endif
 	ret = expand_stack(vma, stack_base);
 	if (ret)