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Date:   Fri, 1 Dec 2023 18:39:18 +0100
From:   Christian Brauner <brauner@kernel.org>
To:     "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Cc:     Serge Hallyn <serge@hallyn.com>, Paul Moore <paul@paul-moore.com>,
        Eric Paris <eparis@redhat.com>,
        James Morris <jmorris@namei.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Amir Goldstein <amir73il@gmail.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-security-module@vger.kernel.org, audit@vger.kernel.org,
        linux-unionfs@vger.kernel.org
Subject: Re: [PATCH 09/16] fs: add vfs_set_fscaps()
Message-ID: <20231201-reintreten-gehalt-435a960f80ed@brauner>
References: <20231129-idmap-fscap-refactor-v1-0-da5a26058a5b@kernel.org>
 <20231129-idmap-fscap-refactor-v1-9-da5a26058a5b@kernel.org>
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On Wed, Nov 29, 2023 at 03:50:27PM -0600, Seth Forshee (DigitalOcean) wrote:
> Provide a type-safe interface for setting filesystem capabilities and a
> generic implementation suitable for most filesystems.
> 
> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> ---
>  fs/xattr.c         | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h |  2 ++
>  2 files changed, 89 insertions(+)
> 
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 3abaf9bef0a5..03cc824e4f87 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -247,6 +247,93 @@ int vfs_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
>  }
>  EXPORT_SYMBOL(vfs_get_fscaps);
>  
> +static int generic_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +			      const struct vfs_caps *caps, int flags)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct vfs_ns_cap_data nscaps;
> +	int size;
> +
> +	size = vfs_caps_to_xattr(idmap, i_user_ns(inode), caps,
> +				 &nscaps, sizeof(nscaps));
> +	if (size < 0)
> +		return size;

I see, here the size is relevant. Ok, but then please make the
lower-level helper use ssize_t.

> +
> +	return __vfs_setxattr_noperm(idmap, dentry, XATTR_NAME_CAPS,
> +				     &nscaps, size, flags);
> +}
> +
> +/**
> + * vfs_set_fscaps - set filesystem capabilities
> + * @idmap: idmap of the mount the inode was found from
> + * @dentry: the dentry on which to set filesystem capabilities
> + * @caps: the filesystem capabilities to be written
> + * @flags: setxattr flags to use when writing the capabilities xattr
> + *
> + * This function writes the supplied filesystem capabilities to the dentry.
> + *
> + * Return: 0 on success, a negative errno on error.
> + */
> +int vfs_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
> +		   const struct vfs_caps *caps, int flags)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct inode *delegated_inode = NULL;
> +	struct vfs_ns_cap_data nscaps;
> +	int size, error;
> +
> +	/*
> +	 * Unfortunately EVM wants to have the raw xattr value to compare to
> +	 * the on-disk version, so we need to pass the raw xattr to the
> +	 * security hooks. But we also want to do security checks before
> +	 * breaking leases, so that means a conversion to the raw xattr here
> +	 * which will usually be reduntant with the conversion we do for
> +	 * writing the xattr to disk.
> +	 */
> +	size = vfs_caps_to_xattr(idmap, i_user_ns(inode), caps, &nscaps,
> +				 sizeof(nscaps));
> +	if (size < 0)
> +		return size;

Oh right, I remember that. Slight eyeroll. See below though...

> +
> +retry_deleg:
> +	inode_lock(inode);
> +
> +	error = xattr_permission(idmap, inode, XATTR_NAME_CAPS, MAY_WRITE);
> +	if (error)
> +		goto out_inode_unlock;
> +	error = security_inode_setxattr(idmap, dentry, XATTR_NAME_CAPS, &nscaps,
> +					size, flags);
> +	if (error)
> +		goto out_inode_unlock;

For posix acls I added dedicated security hooks that take the struct
posix_acl stuff and then plumb that down into the security modules. You
could do the same thing here and then just force EVM and others to do
their own conversion from in-kernel to xattr format, instead of forcing
the VFS to do this.

Because right now we make everyone pay the price all the time when
really EVM should pay that price and this whole unpleasantness.

> +
> +	error = try_break_deleg(inode, &delegated_inode);
> +	if (error)
> +		goto out_inode_unlock;
> +
> +	if (inode->i_opflags & IOP_XATTR) {

So I'm trying to remember the details how I did this for POSIX ACLs in
commit e499214ce3ef ("acl: don't depend on IOP_XATTR"). I think what you
did here is correct because you need to have an xattr handler for
fscaps currently. IOW, it isn't purely based on inode operations.

And here starts the hate mail in so far as you'll hate me for asking
this:

I think I asked this before when we talked about this but how feasible
would it be to move fscaps completely off of xattr handlers and purely
on inode operations for all filesystems?

Yes, that's a fairly large patchset but it would also be a pretty good
win because we avoid munging this from inode operations through xattr
handlers again which seems a bit ugly and what we really wanted to
avoid desperately with POSIX ACLs.

If this is feasible and you'd be up for it I wouldn't even mind doing
that in two steps. IOW, merge something like this first and them move
everyone off of their individual xattr handlers.

Could you quickly remind me whether there would be any issues with this?