Re: [PATCH 3/3] kernfs: Initialize security of newly created nodes

Stephen Smalley <sds@xxxxxxxxxxxxx> · Wed, 9 Jan 2019 10:44:29 -0500

On 1/9/19 4:10 AM, Ondrej Mosnacek wrote:
Use the new security_object_init_security() hook to allow LSMs to
possibly assign a non-default security context to newly created nodes
based on the context of their parent node.

This fixes an issue with cgroupfs under SELinux, where newly created
cgroup subdirectories would not inherit its parent's context if it had
been set explicitly to a non-default value (other than the genfs context
specified by the policy). This can be reproduced as follows:

     # mkdir /sys/fs/cgroup/unified/test
     # chcon -R system_u:object_r:cgroup_t:s0:c123 /sys/fs/cgroup/unified/test
     # ls -lZ /sys/fs/cgroup/unified
     total 0
     -r--r--r--.  1 root root system_u:object_r:cgroup_t:s0      0 Jan  8 05:00 cgroup.controllers
     -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0      0 Jan  8 05:00 cgroup.max.depth
     -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0      0 Jan  8 05:00 cgroup.max.descendants
     -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0      0 Jan  8 05:00 cgroup.procs
     -r--r--r--.  1 root root system_u:object_r:cgroup_t:s0      0 Jan  8 05:00 cgroup.stat
     -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0      0 Jan  8 05:00 cgroup.subtree_control
     -rw-r--r--.  1 root root system_u:object_r:cgroup_t:s0      0 Jan  8 05:00 cgroup.threads
     drwxr-xr-x.  2 root root system_u:object_r:cgroup_t:s0      0 Jan  8 04:54 init.scope
     drwxr-xr-x. 25 root root system_u:object_r:cgroup_t:s0      0 Jan  8 04:54 system.slice
     drwxr-xr-x.  2 root root system_u:object_r:cgroup_t:s0:c123 0 Jan  8 04:59 test
     drwxr-xr-x.  3 root root system_u:object_r:cgroup_t:s0      0 Jan  8 04:55 user.slice
     # mkdir /sys/fs/cgroup/unified/test/subdir

Actual result:

     # ls -ldZ /sys/fs/cgroup/unified/test/subdir
     drwxr-xr-x. 2 root root system_u:object_r:cgroup_t:s0 0 Jan  8 05:10 /sys/fs/cgroup/unified/test/subdir

Expected result:

     # ls -ldZ /sys/fs/cgroup/unified/test/subdir
     drwxr-xr-x. 2 root root unconfined_u:object_r:cgroup_t:s0:c123 0 Jan  8 05:10 /sys/fs/cgroup/unified/test/subdir

Link: https://github.com/SELinuxProject/selinux-kernel/issues/39
Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>

Reviewed-by: Stephen Smalley <sds@xxxxxxxxxxxxx>

---
  fs/kernfs/dir.c             | 49 ++++++++++++++++++++++++++++++++++---
  fs/kernfs/inode.c           |  9 +++----
  fs/kernfs/kernfs-internal.h |  4 +++
  3 files changed, 54 insertions(+), 8 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 4ca0b5c18192..8a678a934f65 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -15,6 +15,7 @@
  #include <linux/slab.h>
  #include <linux/security.h>
  #include <linux/hash.h>
+#include <linux/stringhash.h>
  
  #include "kernfs-internal.h"
  
@@ -617,7 +618,43 @@ struct kernfs_node *kernfs_node_from_dentry(struct dentry *dentry)
  	return NULL;
  }
  
-static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
+static int kernfs_node_init_security(struct kernfs_node *parent,
+				     struct kernfs_node *kn, umode_t mode)
+{
+	struct kernfs_iattrs *attrs;
+	struct qstr q;
+	void *ctx;
+	u32 ctxlen;
+	int ret;
+
+	/* If parent has no explicit context set, leave child unset as well */
+	if (!parent->iattr)
+		return 0;
+	if (!parent->iattr->ia_secdata || !parent->iattr->ia_secdata_len)
+		return 0;
+
+	q.name = kn->name;
+	q.hash_len = hashlen_string(parent, kn->name);
+
+	ret = security_object_init_security(parent->iattr->ia_secdata,
+					    parent->iattr->ia_secdata_len,
+					    &q, (u16)mode, &ctx, &ctxlen);
+	if (ret)
+		return ret;
+
+	attrs = kernfs_iattrs(kn);
+	if (!attrs) {
+		security_release_secctx(ctx, ctxlen);
+		return -ENOMEM;
+	}
+
+	kernfs_node_setsecdata(attrs, &ctx, &ctxlen);
+	/* The inode is fresh, so the returned ctx is always NULL. */
+	return 0;
+}
+
+static struct kernfs_node *__kernfs_new_node(struct kernfs_node *parent,
+					     struct kernfs_root *root,
  					     const char *name, umode_t mode,
  					     kuid_t uid, kgid_t gid,
  					     unsigned flags)
@@ -674,6 +711,12 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
  			goto err_out3;
  	}
  
+	if (parent) {
+		ret = kernfs_node_init_security(parent, kn, mode);
+		if (ret)
+			goto err_out3;
+	}
+
  	return kn;
  
   err_out3:
@@ -692,7 +735,7 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
  {
  	struct kernfs_node *kn;
  
-	kn = __kernfs_new_node(kernfs_root(parent),
+	kn = __kernfs_new_node(parent, kernfs_root(parent),
  			       name, mode, uid, gid, flags);
  	if (kn) {
  		kernfs_get(parent);
@@ -962,7 +1005,7 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
  	INIT_LIST_HEAD(&root->supers);
  	root->next_generation = 1;
  
-	kn = __kernfs_new_node(root, "", S_IFDIR | S_IRUGO | S_IXUGO,
+	kn = __kernfs_new_node(NULL, root, "", S_IFDIR | S_IRUGO | S_IXUGO,
  			       GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
  			       KERNFS_DIR);
  	if (!kn) {
diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index 80cebcd94c90..e6db8d23437b 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -31,7 +31,7 @@ static const struct inode_operations kernfs_iops = {
  	.listxattr	= kernfs_iop_listxattr,
  };
  
-static struct kernfs_iattrs *kernfs_iattrs(struct kernfs_node *kn)
+struct kernfs_iattrs *kernfs_iattrs(struct kernfs_node *kn)
  {
  	static DEFINE_MUTEX(iattr_mutex);
  	struct kernfs_iattrs *ret;
@@ -135,8 +135,8 @@ out:
  	return error;
  }
  
-static int kernfs_node_setsecdata(struct kernfs_iattrs *attrs, void **secdata,
-				  u32 *secdata_len)
+void kernfs_node_setsecdata(struct kernfs_iattrs *attrs, void **secdata,
+			    u32 *secdata_len)
  {
  	void *old_secdata;
  	size_t old_secdata_len;
@@ -149,7 +149,6 @@ static int kernfs_node_setsecdata(struct kernfs_iattrs *attrs, void **secdata,
  
  	*secdata = old_secdata;
  	*secdata_len = old_secdata_len;
-	return 0;
  }
  
  ssize_t kernfs_iop_listxattr(struct dentry *dentry, char *buf, size_t size)
@@ -365,7 +364,7 @@ static int kernfs_security_xattr_set(const struct xattr_handler *handler,
  		return error;
  
  	mutex_lock(&kernfs_mutex);
-	error = kernfs_node_setsecdata(attrs, &secdata, &secdata_len);
+	kernfs_node_setsecdata(attrs, &secdata, &secdata_len);
  	mutex_unlock(&kernfs_mutex);
  
  	if (secdata)
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 3d83b114bb08..f6fb2df24c30 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -92,6 +92,10 @@ int kernfs_iop_getattr(const struct path *path, struct kstat *stat,
  ssize_t kernfs_iop_listxattr(struct dentry *dentry, char *buf, size_t size);
  int __kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr);
  
+struct kernfs_iattrs *kernfs_iattrs(struct kernfs_node *kn);
+void kernfs_node_setsecdata(struct kernfs_iattrs *attrs, void **secdata,
+			    u32 *secdata_len);
+
  /*
   * dir.c
   */








