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

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Date:	Fri, 9 Mar 2012 14:47:32 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	Matt Helsley <matthltc@...ibm.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 03/09, Cyrill Gorcunov wrote:
>
> On Fri, Mar 09, 2012 at 04:57:35PM +0400, Cyrill Gorcunov wrote:
> >
> > yeah, thanks, will update.
> >
>
> This one should fit all requirements I hope.

Oh, sorry Cyrill, I simply can't resist...

> +static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
> +{
> +	struct file *exe_file;
> +	struct dentry *dentry;
> +	int err;
> +
> +	if (mm->num_exe_file_vmas)
> +		return -EBUSY;
> +
> +	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;
> +
> +	err = inode_permission(dentry->d_inode, MAY_EXEC);
> +	if (err)
> +		goto exit;
> +
> +	/*
> +	 * Setting new mm::exe_file is only allowed
> +	 * when no VM_EXECUTABLE vma's left. This is
> +	 * a special C/R case when a restored program
> +	 * need to change own /proc/$pid/exe symlink.
> +	 * After this call mm::num_exe_file_vmas become
> +	 * meaningless. If mm::num_exe_file_vmas will
> +	 * ever increase back from zero -- this code
> +	 * needs to be revised, thus WARN_ here, just
> +	 * to be sure.

To be shure in what??

> +	 */
> +	down_write(&mm->mmap_sem);
> +	WARN_ON_ONCE(mm->num_exe_file_vmas);

We already checked it is zero. Yes, it shouldn't grow. But why
do we need another check here?

If it can grow, it can grow after we drop mmap_sem as well and
this would be wrong. So may be we need another WARN_ON() at the
end?

I'd understand if you add something like

	WARN_ON(!mm->num_exe_file_vmas && !current->in_exec);

into added_exe_file_vma().

Or

	WARN_ON(mm->num_exe_file_vmas <= 0);

into removed_exe_file_vma().

But imho your WARN looks like "OK, I checked it lockless but I
am not sure this is correct".

Oleg.

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