[PATCH] fix use-after-free bug in move_vma()

move_vma() calls do_munmap() and then uses the memory at *new_vma.

But when starting X11 it just happens that the memory which do_munmap
unmapped had the same start address and the range at *new_vma.  So new_vma
is freed by do_munmap().

This was never noticed before because (vm_flags & VM_LOCKED) evaluates
false when vm_flags is 0x5a5a5a5a.  But I just changed that to 0x6b6b6b6b
and boom - we call make_pages_present() with start == end == 0x6b6b6b6b and
it goes BUG.

So I think the right fix here is for move_vma() to not inspect the values
of any vma's after it has called do_munmap().

The patch does that, for `new_vma'.

The local variable `vma' is also being used after the call do do_munmap(),
and this may also be a bug.  Proving that this is not so, and adding a
comment to explain why is hereby added to Hugh's todo list ;)

mm/mremap.c

@@ -227,6 +227,10 @@ static unsigned long move_vma(struct vm_area_struct * vma,
 	}
 
 	if (!move_page_tables(vma, new_addr, addr, old_len)) {
+		unsigned long must_fault_in;
+		unsigned long fault_in_start;
+		unsigned long fault_in_end;
+
 		if (allocated_vma) {
 			*new_vma = *vma;
 			INIT_LIST_HEAD(&new_vma->shared);
@@ -251,19 +255,25 @@ static unsigned long move_vma(struct vm_area_struct * vma,
 		} else
 			vma = NULL;		/* nothing more to do */
 
+		must_fault_in = new_vma->vm_flags & VM_LOCKED;
+		fault_in_start = new_vma->vm_start;
+		fault_in_end = new_vma->vm_end;
+
 		do_munmap(current->mm, addr, old_len);
 
+		/* new_vma could have been invalidated by do_munmap */
+
 		/* Restore VM_ACCOUNT if one or two pieces of vma left */
 		if (vma) {
 			vma->vm_flags |= VM_ACCOUNT;
 			if (split)
 				vma->vm_next->vm_flags |= VM_ACCOUNT;
 		}
+
 		current->mm->total_vm += new_len >> PAGE_SHIFT;
-		if (new_vma->vm_flags & VM_LOCKED) {
+		if (must_fault_in) {
 			current->mm->locked_vm += new_len >> PAGE_SHIFT;
-			make_pages_present(new_vma->vm_start,
-					   new_vma->vm_end);
+			make_pages_present(fault_in_start, fault_in_end);
 		}
 		return new_addr;
 	}