linux-kernel - Re: [PATCH 1/9] exec_path 1/9: introduce ->exec

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Date:	Tue, 16 Jun 2009 02:10:32 +0400
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Helsley <matthltc@...ibm.com>, xemul@...allels.com,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, dave@...ux.vnet.ibm.com,
	mingo@...e.hu, torvalds@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/9] exec_path 1/9: introduce ->exec_path and switch
	/proc/*/exe

On Sat, Jun 06, 2009 at 08:22:44AM +0100, Al Viro wrote:
> On Thu, Jun 04, 2009 at 03:04:22AM +0400, Alexey Dobriyan wrote:
> > diff --git a/fs/binfmt_som.c b/fs/binfmt_som.c
> > index eff74b9..6c56262 100644
> > --- a/fs/binfmt_som.c
> > +++ b/fs/binfmt_som.c
> > @@ -174,6 +174,7 @@ static int map_som_binary(struct file *file,
> >  	up_write(&current->mm->mmap_sem);
> >  	if (retval > 0 || retval < -1024)
> >  		retval = 0;
> > +	set_task_exec_path(current, &bprm->file->f_path);
> 
> Oh?  Even on failure exits?

OK.

> > +	if (!path->mnt || !path->dentry)
> > +		return -ENOENT;
> 
> Umm...  I really don't like that.  Note that path with NULL vfsmount
> and non-NULL dentry should never happen.  If anything, we ought
> to add path_empty(path) (!(path)->mnt) and convert such places to it.

Why mntget/mntput handle NULL vfsmount?

> > +static inline void set_task_exec_path(struct task_struct *tsk, struct path *path)
> > +{
> > +	struct path old_path;
> > +
> > +	path_get(path);
> > +	task_lock(tsk);
> > +	old_path = tsk->exec_path;
> > +	tsk->exec_path = *path;
> > +	task_unlock(tsk);
> > +	path_put(&old_path);
> > +}
> 
> Do we ever have a right to do that to anything other than current?  Note
> that fork() is a special case anyway...

Locking wise? Yes, why not.
 
> > +	set_task_exec_path(tsk, &(struct path){ .mnt = NULL, .dentry = NULL });
> 
> Ew...

:^)

> > +	get_task_exec_path(current, &p->exec_path);
> > +
> 
> We already have that value sitting there, so why not get_path(&p->exec_path)?
> 
> The real problem I have with that we *really* can't umount the filesystem
> that used to host the binary anymore.  At all.

OTOH, you can always answer the question what is executing unless task is
sufficiently dead.

Now, I dont' think anyone unmaps old executable except malicious stuff.

> Frankly, I'm almost tempted to add explicit way to switch the damn thing
> via /proc/self/something - e.g. allow a binary to write a pathname to
> /proc/self/set_exec and have that switch the sucker.  The interesting
> part, of course, is figuring out the security implications of that...

I think nobody will use it.

I think /proc/*/exe should stay informational without task being able
to mangle it at will.
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