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

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Date:	Fri, 9 Mar 2012 13:48:11 +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:
>
> +static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
> +{
> +	struct file *exe_file;
> +	struct dentry *dentry;
> +	int err;
> +
> +	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 executable VMAs left. This is
                   ^^^^^^^^^^
Perhaps this is just me, but imho "executable" is not clear enough.
I'd suggest VM_EXECUTABLE to avoid the confusion with VM_EXEC.

> +	 * 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.
> +	 */
> +	down_write(&mm->mmap_sem);
> +	if (mm->num_exe_file_vmas == 0) {

You can check this at the very start lockless and simplify the code.
Once it is zero, it can never grow (or we have a bug anyway).

> +		set_mm_exe_file(mm, exe_file);
> +		exe_file = NULL;
> +	} else
> +		err = -EBUSY;
> +	up_write(&mm->mmap_sem);
> +
> +exit:
> +	if (exe_file)
> +		fput(exe_file);

This doesn't look correct, you need fput() in any case.
set_mm_exe_file() does another get_file().

Oleg.

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