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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAGXu5jLZ8Ldv4vCjN6+QOa8v=GuUDU9t8sJsTNaQJGYtpdCayA@mail.gmail.com>
Date:	Thu, 10 Dec 2015 10:05:50 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Willy Tarreau <w@....eu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	yalin wang <yalin.wang2010@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing

On Wed, Dec 9, 2015 at 11:06 PM, Willy Tarreau <w@....eu> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/