[6/8] SELinux: Handle opening of a unioned file
diff mbox

Message ID 20150618133302.12722.14996.stgit@warthog.procyon.org.uk
State New
Headers show

Commit Message

David Howells June 18, 2015, 1:33 p.m. UTC
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@redhat.com>
---

 security/selinux/hooks.c          |   69 +++++++++++++++++++++++++++++++++++++
 security/selinux/include/objsec.h |    1 +
 2 files changed, 70 insertions(+)


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Comments

Stephen Smalley June 18, 2015, 2:54 p.m. UTC | #1
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@redhat.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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David Howells June 18, 2015, 3:04 p.m. UTC | #2
Stephen Smalley <sds@tycho.nsa.gov> wrote:

> > +	/* 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.

Actually, this comment should probably be deleted.  I am currently thinking
that access through the overlay fs should only be mediated by the label on the
overlay inode and should not involve the lower inode.

Possibly, then the lower file label should be reckoned against the label of
whoever created the *mount*.

David
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Patch
diff mbox

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);
+			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.
+	 */
+	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 */
 };