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

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Date:	Wed, 14 Mar 2012 09:47:28 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3

On Tue, Mar 13, 2012 at 06:41:06PM -0700, Matt Helsley wrote:
...
> > +
> > +	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 an overall picture.
> > +	 */
> > +	err = -EACCES;
> > +	if (!S_ISREG(dentry->d_inode->i_mode)	||
> > +	    exe_file->f_path.mnt->mnt_flags & MNT_NOEXEC)
> > +		goto exit;
> 
> You could factor out this portion of the access checking from open_exec()
> after the do_filp_open() in open_exec() and re-use it here. I know it's
> tiny helper but tying these two together might be good for
> maintenance later.
> 

Matt, I really dont wanna touch code outside of prctl and this function
in particualar, at least in this patch, ie I can clean up and factor out
is on top of the patch, as a separate task.

> Should it check for some of the flags open_exec() uses? open_exec()
> passes:
> 
> 	O_LARGEFILE|O_RDONLY|__FMODE_EXEC
> 
> to do_filp_open(). I think a O_RDONLY check might be good. I don't
> think __FMODE_EXEC is something userspace can set so could be ignored.
> O_LARGEFILE might be important though.

Well, we're not going to read from this file, so it is not that important
at moment, so previously I've had

> +     if ((exe_file->f_flags & O_ACCMODE) != O_RDONLY)
> +             goto exit;

and Oleg pointed me

 | But the O_RDONLY check looks strange. We are not going to write
 | to this file, we only set the name (and that is why I think it
 | should be mm->exe_path). What is the point to check that the file
 | was opened without FMODE_WRITE? Even if there were any security
 | risk the apllication can open this file again with the different
 | flags.

so I dropped it. And I think the same applies to O_LARGEFILE. Sure
it's not a problem to bring it back but should we?

	Cyrill
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