Re: [PATCH v5] fs: clear file privilege bits when mmap writing

[Kees Cook]

On Wed, Dec 9, 2015 at 11:06 PM, Willy Tarreau <w@xxxxxx> wrote:
> Hi Kees,
>
> Why not add a new file flag instead ?
>
> Something like this (editing your patch by hand to illustrate) :
>
> diff --git a/fs/file_table.c b/fs/file_table.c
> index ad17e05ebf95..3a7eee76ea90 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -191,6 +191,17 @@ static void __fput(struct file *file)
>
>         might_sleep();
>
> +       /*
> +        * XXX: While avoiding mmap_sem, we've already been written to.
> +        * We must ignore the return value, since we can't reject the
> +        * write.
> +        */
> +       if (unlikely(file->f_flags & FL_DROP_PRIVS)) {
> +               mutex_lock(&inode->i_mutex);
> +               file_remove_privs(file);
> +               mutex_unlock(&inode->i_mutex);
> +       }
> +
>         fsnotify_close(file);
>         /*
>          * The function eventpoll_release() should be the first called
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3aa514254161..409bd7047e7e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -913,3 +913,4 @@
>  #define FL_OFDLCK       1024    /* lock is "owned" by struct file */
>  #define FL_LAYOUT       2048    /* outstanding pNFS layout */
> +#define FL_DROP_PRIVS   4096    /* lest something weird decides that 2 is OK */
>
> diff --git a/mm/memory.c b/mm/memory.c
> index c387430f06c3..08a77e0cf65f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2036,6 +2036,7 @@ static inline int wp_page_reuse(struct mm_struct *mm,
>
>                 if (!page_mkwrite)
>                         file_update_time(vma->vm_file);
> +               vma->vm_file->f_flags |= FL_DROP_PRIVS;
>         }
>
>         return VM_FAULT_WRITE;
>
> Willy
>

Is f_flags safe to write like this without holding a lock?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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