Re: [RFC] [PATCH -mm] file caps: make on-disk capabilities future-proof

[Stephen Smalley <sds@tycho.nsa.gov>]

On Mon, 2007-02-19 at 11:01 -0600, Serge E. Hallyn wrote:
> From: Serge E. Hallyn <serue@us.ibm.com>
> Subject: [PATCH -mm] file caps: make on-disk capabilities future-proof
> 
> Stephen Smalley has pointed out that the current file capabilities
> will eventually pose a problem.
> 
> As the capability set changes and distributions start tagging
> binaries with capabilities, we would like for running an older
> kernel to not necessarily make those binaries unusable.  To
> that end,
> 
> 	1. If capabilities are specified which we don't know
> 	   about, just ignore them, do not return -EPERM as we
> 	   were doing before.

I didn't advocate that change - it is a separate issue from allowing the
capability bitmaps to grow in size in a backward compatible manner.  In
the one case, you have a binary that needs a capability that is unknown
to the kernel, so running it could lead to unexpected failure.  In the
other case, you simply have a binary labeled by newer userspace with a
newer on-disk representation that supports larger bitmaps, but the
binary might only have capabilities set that are known to the kernel.

> 	2. Specify a size with the on-disk capability implementation.
> 	   In this implementation the size is the number of __u32's
> 	   used for each of (eff,perm,inh).  For now, sz is 1.
> 	   When we move to 64-bit capabilities, it becomes 2.

You could alternatively split them into separate xattrs, e.g.
security.cap.eff, security.cap.perm, security.cap.inh, and determine the
bitmap size from the xattr length rather than a separate field.

> diff --git a/security/commoncap.c b/security/commoncap.c
> index be86acb..dc8bf4f 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c

> @@ -148,50 +145,65 @@ static int set_file_caps(struct linux_bi
>  {
>  	struct dentry *dentry;
>  	ssize_t rc;
> -	struct vfs_cap_data_disk dcaps;
> +	struct vfs_cap_data_disk *dcaps;
>  	struct vfs_cap_data caps;
>  	struct inode *inode;
> -	int err;
>  
>  	if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID)
>  		return 0;
>  
>  	dentry = dget(bprm->file->f_dentry);
>  	inode = dentry->d_inode;
> -	if (!inode->i_op || !inode->i_op->getxattr) {
> -		dput(dentry);
> -		return 0;
> +	rc = 0;
> +	if (!inode->i_op || !inode->i_op->getxattr)
> +		goto out;
> +
> +	rc = inode->i_op->getxattr(dentry, XATTR_NAME_CAPS, NULL, 0);
> +	if (rc == -ENODATA) {
> +		rc = 0;
> +		goto out;
> +	}

I'd allocate an initial buffer with an expected size and try first to
avoid always having to make the two ->getxattr calls in the common case.

> +	if (rc < 0)
> +		goto out;
> +	if (rc < sizeof(struct vfs_cap_data_disk)) {

You could make this a bit stricter, as you know that it will have at
least three additional u32 values beyond the header.

> +		rc = -EINVAL;
> +		goto out;
> +	}
> +
> +	dcaps = kmalloc(rc, GFP_KERNEL);
> +	if (!dcaps) {
> +		rc = -ENOMEM;
> +		goto out;
> +	}
> +	rc = inode->i_op->getxattr(dentry, XATTR_NAME_CAPS, dcaps,
> +						XATTR_CAPS_SZ);

I'm confused - you just asked for the actual length of the xattr and
allocated a buffer for it, and then don't use the length in this second
call to ->getxattr.  And since you said you were organizing it as
eff[0..sz-1],perm[0..sz-1],inh[0..sz-1], you do need to read the entire
thing to get all three values even if you only use the lower 32 bits of
each.  Or if you change the organization to avoid the need to read the
entire thing, you don't need the first getxattr call at all, and you
need to change how cap_from_disk extracts the values.

-- 
Stephen Smalley
National Security Agency

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