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Date: Thu, 18 Jun 2015 10:54:17 -0400
From: Stephen Smalley <sds@...ho.nsa.gov>
To: David Howells <dhowells@...hat.com>, viro@...iv.linux.org.uk,
miklos@...redi.hu
CC: linux-fsdevel@...r.kernel.org,
linux-security-module@...r.kernel.org,
linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org,
Selinux@...ho.nsa.gov, Paul Moore <paul@...l-moore.com>
Subject: Re: [PATCH 6/8] SELinux: Handle opening of a unioned file
On 06/18/2015 09:33 AM, David Howells wrote:
> Handle the opening of a unioned file by trying to derive the label that would
> be attached to the union-layer inode if it doesn't exist.
>
> If the union-layer inode does exist (as it necessarily does in overlayfs, but
> not in unionmount), we assume that it has the right label and use that.
> Otherwise we try to get it from the superblock.
>
> If the superblock has a globally-applied label, we use that, otherwise we try
> to transition to an appropriate label. This union label is then stored in the
> file_security_struct.
>
> We then perform an additional check to make sure that the calling task is
> granted permission by the union-layer inode label to open the file in addition
> to a check to make sure that the task is granted permission to open the lower
> file with the lower inode label.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> ---
>
> security/selinux/hooks.c | 69 +++++++++++++++++++++++++++++++++++++
> security/selinux/include/objsec.h | 1 +
> 2 files changed, 70 insertions(+)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index c5d893e2ff23..c4495a797eb1 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3490,10 +3490,72 @@ static int selinux_file_receive(struct file *file)
> return file_has_perm(cred, file, file_to_av(file));
> }
>
> +/*
> + * We have a file opened on a unioned file system that falls through to a file
> + * on a lower layer. If there is a union inode, we try to get the label from
> + * that, otherwise we need to get it from the superblock.
> + *
> + * file->f_path points to the union layer and file->f_inode points to the lower
> + * layer.
> + */
> +static int selinux_file_open_union(struct file *file,
> + struct file_security_struct *fsec,
> + const struct cred *cred)
> +{
> + const struct superblock_security_struct *sbsec;
> + const struct inode_security_struct *isec, *dsec, *fisec;
> + const struct task_security_struct *tsec = current_security();
> + struct common_audit_data ad;
> + struct dentry *union_dentry = file->f_path.dentry;
> + const struct inode *union_inode = d_inode(union_dentry);
> + const struct inode *lower_inode = file_inode(file);
> + struct dentry *dir;
> + int rc;
> +
> + sbsec = union_dentry->d_sb->s_security;
> +
> + if (union_inode) {
> + isec = union_inode->i_security;
> + fsec->union_isid = isec->sid;
> + } else if ((sbsec->flags & SE_SBINITIALIZED) &&
> + (sbsec->behavior == SECURITY_FS_USE_MNTPOINT)) {
> + fsec->union_isid = sbsec->mntpoint_sid;
> + } else {
> + dir = dget_parent(union_dentry);
> + dsec = d_inode(dir)->i_security;
> +
> + rc = security_transition_sid(
> + tsec->sid, dsec->sid,
> + inode_mode_to_security_class(lower_inode->i_mode),
> + &union_dentry->d_name,
> + &fsec->union_isid);
> + dput(dir);
> + if (rc) {
> + pr_warn("%s: security_transition_sid failed, rc=%d (name=%pD)\n",
> + __func__, -rc, file);
I would drop this pr_warn altogether (and ultimately the printk from
inode_init_security). Not necessary.
> + return rc;
> + }
> + }
> +
> + /* We need to check that the union file is allowed to be opened as well
> + * as checking that the lower file is allowed to be opened.
Hmm...so if I try to open a file for write access, then we are going to
require that the process be allowed to write to both the union/overlay
inode and to the lower inode? That seems problematic for the containers
use case where no write access will be granted to the lower files.
> + */
> + if (unlikely(IS_PRIVATE(lower_inode)))
> + return 0;
> +
> + ad.type = LSM_AUDIT_DATA_PATH;
> + ad.u.path = file->f_path;
> +
> + fisec = lower_inode->i_security;
> + return avc_has_perm(cred_sid(cred), fsec->union_isid, fisec->sclass,
> + open_file_to_av(file), &ad);
> +}
> +
> static int selinux_file_open(struct file *file, const struct cred *cred)
> {
> struct file_security_struct *fsec;
> struct inode_security_struct *isec;
> + int rc;
>
> fsec = file->f_security;
> isec = file_inode(file)->i_security;
> @@ -3514,6 +3576,13 @@ static int selinux_file_open(struct file *file, const struct cred *cred)
> * new inode label or new policy.
> * This check is not redundant - do not remove.
> */
> +
> + if (d_inode(file->f_path.dentry) != file->f_inode) {
> + rc = selinux_file_open_union(file, fsec, cred);
> + if (rc < 0)
> + return rc;
> + }
> +
> return file_path_has_perm(cred, file, open_file_to_av(file));
> }
>
> diff --git a/security/selinux/include/objsec.h b/security/selinux/include/objsec.h
> index 81fa718d5cb3..f088c080aa9e 100644
> --- a/security/selinux/include/objsec.h
> +++ b/security/selinux/include/objsec.h
> @@ -54,6 +54,7 @@ struct file_security_struct {
> u32 sid; /* SID of open file description */
> u32 fown_sid; /* SID of file owner (for SIGIO) */
> u32 isid; /* SID of inode at the time of file open */
> + u32 union_isid; /* SID of would-be inodes in union top (or 0) */
> u32 pseqno; /* Policy seqno at the time of file open */
> };
>
>
>
--
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