Re: [PATCH v3 04/38] fs: add mount_setattr()

From: Christoph Hellwig <hch@lst.de>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Lennart Poettering <lennart@poettering.net>,
	Mimi Zohar <zohar@linux.ibm.com>,
	David Howells <dhowells@redhat.com>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	containers@lists.linux-foundation.org,
	Christoph Hellwig <hch@lst.de>, Tycho Andersen <tycho@tycho.ws>,
	Paul Moore <paul@paul-moore.com>,
	Jonathan Corbet <corbet@lwn.net>,
	smbarber@chromium.org, Christoph Hellwig <hch@infradead.org>,
	linux-ext4@vger.kernel.org, Mrunal Patel <mpatel@redhat.com>,
	Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
	selinux@vger.kernel.org, Josh Triplett <josh@joshtriplett.org>,
	linux-fsdevel@vger.kernel.org, Aleksa Sarai <cyphar@cyphar.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>,
	OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
	Geoffrey Thomas <geofft@ldpreload.com>,
	James Bottomley <James.Bottomley@hansenpartnership.com>,
	John Johansen <john.johansen@canonical.com>,
	Theodore Tso <tytso@mit.edu>,
	Seth Forshee <seth.forshee@canonical.com>,
	Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	fstests@vger.kernel.org, linux-security-module@vger.kernel.org,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-api@vger.kernel.org,
	Casey Schaufler <casey@schaufler-ca.com>,
	Alban Crequy <alban@kinvolk.io>,
	linux-integrity@vger.kernel.org, Todd Kjos <tkjos@google.com>
Subject: Re: [PATCH v3 04/38] fs: add mount_setattr()
Date: Wed, 2 Dec 2020 10:55:47 +0100	[thread overview]
Message-ID: <20201202095547.GA6408@lst.de> (raw)
In-Reply-To: <20201202094751.GA6129@lst.de>

On Wed, Dec 02, 2020 at 10:47:51AM +0100, Christoph Hellwig wrote:
> On Wed, Dec 02, 2020 at 10:42:18AM +0100, Christian Brauner wrote:
> > > > +	if (upper_32_bits(attr->attr_set))
> > > > +		return -EINVAL;
> > > > +	if (build_attr_flags(lower_32_bits(attr->attr_set), &kattr->attr_set))
> > > > +		return -EINVAL;
> > > > +
> > > > +	if (upper_32_bits(attr->attr_clr))
> > > > +		return -EINVAL;
> > > > +	if (build_attr_flags(lower_32_bits(attr->attr_clr), &kattr->attr_clr))
> > > > +		return -EINVAL;
> > > 
> > > What is so magic about the upper and lower 32 bits?
> > 
> > Nothing apart from the fact that they arent't currently valid. I can
> > think about reworking these lines. Or do you already have a preferred
> > way of doing this in mind?
> 
> Just turn the attr_flags argument to build_attr_flags into a u64 and
> the first sanity check there will catch all invalid flags, no matter
> where they are places.  That should also generate more efficient code.

And while we're at it:  the check for valid flags in the current
code is a little weird, given that build_attr_flags checks for
them, and but sys_fsmount also has its own slightly narrower checks.

I think it might make sense to split the validity check out of
build_attr_flags. E.g. something like:

static unsigned int attr_flags_to_mnt_flags(u64 attr_flags)
{
	unsigned int mnt_flags = 0;

	if (attr_flags & MOUNT_ATTR_RDONLY)
		mnt_flags |= MNT_READONLY;
	if (attr_flags & MOUNT_ATTR_NOSUID)
		mnt_flags |= MNT_NOSUID;
	if (attr_flags & MOUNT_ATTR_NODEV)
		mnt_flags |= MNT_NODEV;
	if (attr_flags & MOUNT_ATTR_NOEXEC)
		mnt_flags |= MNT_NOEXEC;
	if (attr_flags & MOUNT_ATTR_NODIRATIME)
		mnt_flags |= MNT_NODIRATIME;

	return mnt_flags;
}

#define MOUNT_SETATTR_VALID_FLAGS \
	(MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID | MOUNT_ATTR_NODEV | \
	 MOUNT_ATTR_NOEXEC | MOUNT_ATTR__ATIME | MOUNT_ATTR_NODIRATIME | \
	 MOUNT_ATTR_IDMAP)

static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
			     struct mount_kattr *kattr, unsigned int flags)
{
	...

	if ((attr->attr_set | attr->attr_clr) & ~MOUNT_SETATTR_VALID_FLAGS)
		return -EINVAL;
	kattr->attr_set = attr_flags_to_mnt_flags(attr->attr_set);
	kattr->attr_clr = attr_flags_to_mnt_flags(attr->attr_clr);
	...
}
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