linux-kernel - Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe

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Message-ID: <CAGXu5j+4RRGjzVxLSyz0CZCNMj4dPOxJp3pYGSN_sXFsiMhUaQ@mail.gmail.com>
Date:	Thu, 8 Mar 2012 11:31:58 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>
Subject: Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3

On Thu, Mar 8, 2012 at 8:51 AM, Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
> When we do restore we would like to have a way to setup
> a former mm_struct::exe_file so that /proc/pid/exe would
> point to the original executable file a process had at
> checkpoint time.
>
> For this the PR_SET_MM_EXE_FILE code is introduced.
> This option takes a file descriptor which will be
> set as new /proc/$pid/exe symlink.
>
> This feature is available iif CONFIG_CHECKPOINT_RESTORE is set.
> [...]
> Index: linux-2.6.git/kernel/sys.c
> ===================================================================
> --- linux-2.6.git.orig/kernel/sys.c
> +++ linux-2.6.git/kernel/sys.c
> [...]
> +       exe_file = fget(fd);
> +       if (!exe_file)
> +               return -EBADF;
> +
> +       dentry = exe_file->f_path.dentry;
> +
> +       /*
> +        * Because the original mm->exe_file
> +        * points to executable file, make sure
> +        * this one is executable as well to not
> +        * break "big" picture and proc/pid/exe
> +        * symlink will be still pointing to
> +        * executable one.
> +        */
> +       err = -EACCES;
> +       if (!S_ISREG(dentry->d_inode->i_mode)   ||
> +           exe_file->f_path.mnt->mnt_flags & MNT_NOEXEC)
> +               goto exit;

I'm starting to notice that this pattern (testing ISREG and
MNT_NOEXEC) is getting repeated a few times in the kernel, and at
least the no-new-privs patch (not yet in -mm but hopefully soon given
the seccomp_filter work) updates this pattern everywhere. Perhaps this
should be extracted into a helper first, and then this patch can call
that helper here? (And then nnp can just update the single helper.)

-Kees

-- 
Kees Cook
ChromeOS Security
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