Re: [patch 3/3] mm: variable length argument support

Dan Aloni
2007-08-22T08:49:20Z

On Wed, Jun 13, 2007 at 12:03:37PM +0200, Peter Zijlstra wrote:
> From: Ollie Wild <aaw@google.com>
>
> Remove the arg+env limit of MAX_ARG_PAGES by copying the strings directly
> from the old mm into the new mm.
>
[...]
> +static int __bprm_mm_init(struct linux_binprm *bprm)
> +{
[...]
> + vma->vm_flags = VM_STACK_FLAGS;
> + vma->vm_page_prot = protection_map[vma->vm_flags & 0x7];
> + err = insert_vm_struct(mm, vma);
> + if (err) {
> + up_write(&mm->mmap_sem);
> + goto err;
> + }
> +

That change causes a crash in khelper when overcommit_memory = 2
under 2.6.23-rc3.

When a khelper execs, at __bprm_mm_init() current->mm is still NULL.
insert_vm_struct() calls security_vm_enough_memory(), which calls
__vm_enough_memory(), and that's where current->mm->total_vm gets
dereferenced.

Signed-off-by: Dan Aloni <da-x@monatomic.org>

diff --git a/mm/mmap.c b/mm/mmap.c
index 906ed40..6e021df 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -163,10 +163,12 @@ int __vm_enough_memory(long pages, int cap_sys_admin)
if (!cap_sys_admin)
allowed -= allowed / 32;
allowed += total_swap_pages;
-
- /* Don't let a single process grow too big:
- leave 3% of the size of this process for other processes */
- allowed -= current->mm->total_vm / 32;
+
+ if (current->mm) {
+ /* Don't let a single process grow too big:
+ leave 3% of the size of this process for other processes */
+ allowed -= current->mm->total_vm / 32;
+ }

/*
* cast `allowed' as a signed long because vm_committed_space

--
Dan Aloni
XIV LTD, http://www.xivstorage.com
da-x (at) monatomic.org, dan (at) xiv.co.il