Re: [PATCH] audit: fix audit_exe_compare using get_mm_exe_file

From: Mateusz Guzik
Date: Mon Aug 22 2016 - 12:27:21 EST


On Mon, Aug 22, 2016 at 11:41:34AM -0400, Richard Guy Briggs wrote:
> Fix original naive attempt to get/lock access to task->mm->exe_file by
> using get_mm_exe_file and checking for NULL.
>
> See: https://lkml.org/lkml/2016/7/30/97
>
> Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> ---
> kernel/audit_watch.c | 13 ++++++++++---
> 1 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
> index d6709eb..0b29279 100644
> --- a/kernel/audit_watch.c
> +++ b/kernel/audit_watch.c
> @@ -19,6 +19,7 @@
> * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
> */
>
> +#include <linux/file.h>
> #include <linux/kernel.h>
> #include <linux/audit.h>
> #include <linux/kthread.h>
> @@ -540,14 +541,20 @@ int audit_dupe_exe(struct audit_krule *new, struct audit_krule *old)
>
> int audit_exe_compare(struct task_struct *tsk, struct audit_fsnotify_mark *mark)
> {
> + struct mm_struct *mm;
> struct file *exe_file;
> unsigned long ino;
> dev_t dev;
>
> - rcu_read_lock();
> - exe_file = rcu_dereference(tsk->mm->exe_file);
> + mm = get_task_mm(tsk);
> + if (!mm)
> + return 0;
> + exe_file = get_mm_exe_file(mm);
> + mmput(mm);
> + if (!exe_file)
> + return 0;
> ino = exe_file->f_inode->i_ino;
> dev = exe_file->f_inode->i_sb->s_dev;
> - rcu_read_unlock();
> + fput(exe_file);
> return audit_mark_compare(mark, ino, dev);
> }

I think this works but I have a better idea. This basically duplicates a
part of proc_exe_link and does unnecessary work.

Instead, one can introduce get_task_exe_file. task_lock held guarantees
the stability of mm struct itself, and then we can we can try to grab
the file without mm refcount dance.

I'll implement this later today.

--
Mateusz Guzik