linux-kernel - Re: [PATCH v25 12/12] LRNG - add interface for gathering of raw entropy

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Message-ID: <87a78pl8xp.fsf@suse.de>
Date:   Thu, 21 Nov 2019 13:18:10 +0100
From:   Nicolai Stange <nstange@...e.de>
To:     Stephan Müller <smueller@...onox.de>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        "Theodore Y. Ts'o" <tytso@....edu>, Willy Tarreau <w@....eu>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Vito Caputo <vcaputo@...garu.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>, Ray Strode <rstrode@...hat.com>,
        William Jon McCann <mccann@....edu>,
        zhangjs <zachary@...shancloud.com>,
        Andy Lutomirski <luto@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Nicolai Stange <nstange@...e.de>,
        "Peter\, Matthias" <matthias.peter@....bund.de>,
        Marcelo Henrique Cerri <marcelo.cerri@...onical.com>,
        Roman Drahtmueller <draht@...altsekun.de>,
        Neil Horman <nhorman@...hat.com>
Subject: Re: [PATCH v25 12/12] LRNG - add interface for gathering of raw entropy

Hi Stephan,

two general remarks on debugfs usage below

Stephan Müller <smueller@...onox.de> writes:

> diff --git a/drivers/char/lrng/lrng_testing.c b/drivers/char/lrng/lrng_testing.c
> new file mode 100644
> index 000000000000..5c33d3bd2172
> --- /dev/null
> +++ b/drivers/char/lrng/lrng_testing.c

<snip>


> +/*
> + * This data structure holds the dentry's of the debugfs files establishing
> + * the interface to user space.
> + */
> +struct lrng_raw_debugfs {
> +	struct dentry *lrng_raw_debugfs_root; /* root dentry */
> +	struct dentry *lrng_raw_debugfs_lrng_raw; /* .../lrng_raw */
> +};
> +
> +static struct lrng_raw_debugfs lrng_raw_debugfs;
> +
> +/* DebugFS operations and definition of the debugfs files */
> +static ssize_t lrng_raw_read(struct file *file, char __user *to,
> +			     size_t count, loff_t *ppos)
> +{
> +	loff_t pos = *ppos;
> +	int ret;
> +
> +	if (!count)
> +		return 0;
> +	lrng_raw_entropy_init();
> +	ret = lrng_raw_extract_user(to, count);
> +	lrng_raw_entropy_fini();
> +	if (ret < 0)
> +		return ret;
> +	count -= ret;
> +	*ppos = pos + count;
> +	return ret;
> +}
> +
> +/* Module init: allocate memory, register the debugfs files */
> +static int lrng_raw_debugfs_init(void)
> +{
> +	lrng_raw_debugfs.lrng_raw_debugfs_root =
> +		debugfs_create_dir(KBUILD_MODNAME, NULL);
> +	if (IS_ERR(lrng_raw_debugfs.lrng_raw_debugfs_root)) {
> +		lrng_raw_debugfs.lrng_raw_debugfs_root = NULL;
> +		return PTR_ERR(lrng_raw_debugfs.lrng_raw_debugfs_root);
> +	}

I think pointers returned by the debugfs API are not supposed to get
checked for NULL/IS_ERR(), c.f commit ff9fb72bc077 ("debugfs: return
error values, not NULL") or the the output from

  git log --pretty=oneline | grep 'no need to check return value of debugfs_create'

(Also the above code is dubious: you're effectively returning
 PTR_ERR(NULL)).



> +	return 0;
> +}
> +
> +static struct file_operations lrng_raw_name_fops = {
> +	.owner = THIS_MODULE,
> +	.read = lrng_raw_read,
> +};
> +
> +static int lrng_raw_debugfs_init_name(void)
> +{
> +	lrng_raw_debugfs.lrng_raw_debugfs_lrng_raw =
> +		debugfs_create_file("lrng_raw", 0400,
> +				    lrng_raw_debugfs.lrng_raw_debugfs_root,
> +				    NULL, &lrng_raw_name_fops);q

CONFIG_LRNG_TESTING is a bool and thus, this debugfs file can't ever get
removed. Even if it could, this inode hasn't got any data associated
with it and so file removal would not be a problem for lrng_raw_read().

Please consider using debugfs_create_file_unsafe() instead to save
debugfs from kmalloc()ing a proxy file_operations protecting your fops
against concurrent file removal.



> +	if (IS_ERR(lrng_raw_debugfs.lrng_raw_debugfs_lrng_raw)) {
> +		lrng_raw_debugfs.lrng_raw_debugfs_lrng_raw = NULL;
> +		return PTR_ERR(lrng_raw_debugfs.lrng_raw_debugfs_lrng_raw);
> +	}

Same comment regarding return value checking applies here.

Thanks,

Nicolai


> +	return 0;
> +}
> +
> +static int __init lrng_raw_init(void)
> +{
> +	int ret = lrng_raw_debugfs_init();
> +
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = lrng_raw_debugfs_init_name();
> +	if (ret < 0)
> +		debugfs_remove_recursive(
> +					lrng_raw_debugfs.lrng_raw_debugfs_root);
> +
> +	return ret;
> +}
> +
> +static void __exit lrng_raw_exit(void)
> +{
> +	debugfs_remove_recursive(lrng_raw_debugfs.lrng_raw_debugfs_root);
> +}
> +
> +module_init(lrng_raw_init);
> +module_exit(lrng_raw_exit);
> +
> +MODULE_LICENSE("Dual BSD/GPL");
> +MODULE_AUTHOR("Stephan Mueller <smueller@...onox.de>");
> +MODULE_DESCRIPTION("Kernel module for gathering raw entropy");

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