Re: [PATCH 4/4] selinux: Convert isec->lock into a spinlock

[Andreas Gruenbacher <agruenba@redhat.com>]

On Mon, Nov 14, 2016 at 11:22 PM, Paul Moore <paul@paul-moore.com> wrote:
> We shouldn't need the spinlocks on the socket_post_create() and the
> socket_accept() hooks as the callers should still have exclusive
> access to the socket/inode at that point.
>
> I didn't check all the callers of the inode_init_security(), but it
> looks like the same idea applies.

Indeed.  An updated patch with the unnecessary locking removed follows.

Thanks,
Andreas

--

Convert isec->lock from a mutex into a spinlock.  Instead of holding the
lock while sleeping in inode_doinit_with_dentry, set isec->initialized
to LABEL_PENDING and release the lock.  Then, when the sid has been
determined, re-acquire the lock.  If isec->initialized is still set to
LABEL_PENDING, set isec->sid; otherwise, the sid has been set by another
task (LABEL_INITIALIZED) or invalidated (LABEL_INVALID) in the meantime.

This fixes a deadlock on gfs2 where

 * one task is in inode_doinit_with_dentry -> gfs2_getxattr, holds
   isec->lock, and tries to acquire the inode's glock, and

 * another task is in do_xmote -> inode_go_inval ->
   selinux_inode_invalidate_secctx, holds the inode's glock, and tries
   to acquire isec->lock.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 security/selinux/hooks.c          | 102 +++++++++++++++++++++++---------------
 security/selinux/include/objsec.h |   5 +-
 2 files changed, 66 insertions(+), 41 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index cf5067e..ea77ed3 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -231,7 +231,7 @@ static int inode_alloc_security(struct inode *inode)
 	if (!isec)
 		return -ENOMEM;
 
-	mutex_init(&isec->lock);
+	spin_lock_init(&isec->lock);
 	INIT_LIST_HEAD(&isec->list);
 	isec->inode = inode;
 	isec->sid = SECINITSID_UNLABELED;
@@ -1381,7 +1381,8 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 {
 	struct superblock_security_struct *sbsec = NULL;
 	struct inode_security_struct *isec = inode->i_security;
-	u32 sid;
+	u32 task_sid, sid = 0;
+	u16 sclass;
 	struct dentry *dentry;
 #define INITCONTEXTLEN 255
 	char *context = NULL;
@@ -1391,7 +1392,7 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 	if (isec->initialized == LABEL_INITIALIZED)
 		return 0;
 
-	mutex_lock(&isec->lock);
+	spin_lock(&isec->lock);
 	if (isec->initialized == LABEL_INITIALIZED)
 		goto out_unlock;
 
@@ -1410,12 +1411,18 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 		goto out_unlock;
 	}
 
+	sclass = isec->sclass;
+	task_sid = isec->task_sid;
+	sid = isec->sid;
+	isec->initialized = LABEL_PENDING;
+	spin_unlock(&isec->lock);
+
 	switch (sbsec->behavior) {
 	case SECURITY_FS_USE_NATIVE:
 		break;
 	case SECURITY_FS_USE_XATTR:
 		if (!(inode->i_opflags & IOP_XATTR)) {
-			isec->sid = sbsec->def_sid;
+			sid = sbsec->def_sid;
 			break;
 		}
 		/* Need a dentry, since the xattr API requires one.
@@ -1437,7 +1444,7 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 			 * inode_doinit with a dentry, before these inodes could
 			 * be used again by userspace.
 			 */
-			goto out_unlock;
+			goto out;
 		}
 
 		len = INITCONTEXTLEN;
@@ -1445,7 +1452,7 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 		if (!context) {
 			rc = -ENOMEM;
 			dput(dentry);
-			goto out_unlock;
+			goto out;
 		}
 		context[len] = '\0';
 		rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX, context, len);
@@ -1456,14 +1463,14 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 			rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX, NULL, 0);
 			if (rc < 0) {
 				dput(dentry);
-				goto out_unlock;
+				goto out;
 			}
 			len = rc;
 			context = kmalloc(len+1, GFP_NOFS);
 			if (!context) {
 				rc = -ENOMEM;
 				dput(dentry);
-				goto out_unlock;
+				goto out;
 			}
 			context[len] = '\0';
 			rc = __vfs_getxattr(dentry, inode, XATTR_NAME_SELINUX, context, len);
@@ -1475,7 +1482,7 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 				       "%d for dev=%s ino=%ld\n", __func__,
 				       -rc, inode->i_sb->s_id, inode->i_ino);
 				kfree(context);
-				goto out_unlock;
+				goto out;
 			}
 			/* Map ENODATA to the default file SID */
 			sid = sbsec->def_sid;
@@ -1505,28 +1512,25 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 			}
 		}
 		kfree(context);
-		isec->sid = sid;
 		break;
 	case SECURITY_FS_USE_TASK:
-		isec->sid = isec->task_sid;
+		sid = task_sid;
 		break;
 	case SECURITY_FS_USE_TRANS:
 		/* Default to the fs SID. */
-		isec->sid = sbsec->sid;
+		sid = sbsec->sid;
 
 		/* Try to obtain a transition SID. */
-		rc = security_transition_sid(isec->task_sid, sbsec->sid,
-					     isec->sclass, NULL, &sid);
+		rc = security_transition_sid(task_sid, sid, sclass, NULL, &sid);
 		if (rc)
-			goto out_unlock;
-		isec->sid = sid;
+			goto out;
 		break;
 	case SECURITY_FS_USE_MNTPOINT:
-		isec->sid = sbsec->mntpoint_sid;
+		sid = sbsec->mntpoint_sid;
 		break;
 	default:
 		/* Default to the fs superblock SID. */
-		isec->sid = sbsec->sid;
+		sid = sbsec->sid;
 
 		if ((sbsec->flags & SE_SBGENFS) && !S_ISLNK(inode->i_mode)) {
 			/* We must have a dentry to determine the label on
@@ -1549,21 +1553,29 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 			 * could be used again by userspace.
 			 */
 			if (!dentry)
-				goto out_unlock;
-			rc = selinux_genfs_get_sid(dentry, isec->sclass,
-						   sbsec->flags, &sid);
+				goto out;
+			rc = selinux_genfs_get_sid(dentry, sclass, sbsec->flags, &sid);
 			dput(dentry);
 			if (rc)
-				goto out_unlock;
-			isec->sid = sid;
+				goto out;
 		}
 		break;
 	}
 
-	isec->initialized = LABEL_INITIALIZED;
+out:
+	spin_lock(&isec->lock);
+	if (isec->initialized == LABEL_PENDING) {
+		if (!sid || rc) {
+			isec->initialized = LABEL_INVALID;
+			goto out_unlock;
+		}
+
+		isec->initialized = LABEL_INITIALIZED;
+		isec->sid = sid;
+	}
 
 out_unlock:
-	mutex_unlock(&isec->lock);
+	spin_unlock(&isec->lock);
 	return rc;
 }
 
@@ -3194,9 +3206,11 @@ static void selinux_inode_post_setxattr(struct dentry *dentry, const char *name,
 	}
 
 	isec = backing_inode_security(dentry);
+	spin_lock(&isec->lock);
 	isec->sclass = inode_mode_to_security_class(inode->i_mode);
 	isec->sid = newsid;
 	isec->initialized = LABEL_INITIALIZED;
+	spin_unlock(&isec->lock);
 
 	return;
 }
@@ -3289,9 +3303,11 @@ static int selinux_inode_setsecurity(struct inode *inode, const char *name,
 	if (rc)
 		return rc;
 
+	spin_lock(&isec->lock);
 	isec->sclass = inode_mode_to_security_class(inode->i_mode);
 	isec->sid = newsid;
 	isec->initialized = LABEL_INITIALIZED;
+	spin_unlock(&isec->lock);
 	return 0;
 }
 
@@ -3952,9 +3968,11 @@ static void selinux_task_to_inode(struct task_struct *p,
 	struct inode_security_struct *isec = inode->i_security;
 	u32 sid = task_sid(p);
 
+	spin_lock(&isec->lock);
 	isec->sclass = inode_mode_to_security_class(inode->i_mode);
 	isec->sid = sid;
 	isec->initialized = LABEL_INITIALIZED;
+	spin_unlock(&isec->lock);
 }
 
 /* Returns error only if unable to parse addresses */
@@ -4273,24 +4291,24 @@ static int selinux_socket_post_create(struct socket *sock, int family,
 	const struct task_security_struct *tsec = current_security();
 	struct inode_security_struct *isec = inode_security_novalidate(SOCK_INODE(sock));
 	struct sk_security_struct *sksec;
+	u16 sclass = socket_type_to_security_class(family, type, protocol);
+	u32 sid = SECINITSID_KERNEL;
 	int err = 0;
 
-	isec->sclass = socket_type_to_security_class(family, type, protocol);
-
-	if (kern)
-		isec->sid = SECINITSID_KERNEL;
-	else {
-		err = socket_sockcreate_sid(tsec, isec->sclass, &(isec->sid));
+	if (!kern) {
+		err = socket_sockcreate_sid(tsec, sclass, &sid);
 		if (err)
 			return err;
 	}
 
+	isec->sclass = sclass;
+	isec->sid = sid;
 	isec->initialized = LABEL_INITIALIZED;
 
 	if (sock->sk) {
 		sksec = sock->sk->sk_security;
-		sksec->sid = isec->sid;
-		sksec->sclass = isec->sclass;
+		sksec->sclass = sclass;
+		sksec->sid = sid;
 		err = selinux_netlbl_socket_post_create(sock->sk, family);
 	}
 
@@ -4466,16 +4484,22 @@ static int selinux_socket_accept(struct socket *sock, struct socket *newsock)
 	int err;
 	struct inode_security_struct *isec;
 	struct inode_security_struct *newisec;
+	u16 sclass;
+	u32 sid;
 
 	err = sock_has_perm(current, sock->sk, SOCKET__ACCEPT);
 	if (err)
 		return err;
 
-	newisec = inode_security_novalidate(SOCK_INODE(newsock));
-
 	isec = inode_security_novalidate(SOCK_INODE(sock));
-	newisec->sclass = isec->sclass;
-	newisec->sid = isec->sid;
+	spin_lock(&isec->lock);
+	sclass = isec->sclass;
+	sid = isec->sid;
+	spin_unlock(&isec->lock);
+
+	newisec = inode_security_novalidate(SOCK_INODE(newsock));
+	newisec->sclass = sclass;
+	newisec->sid = sid;
 	newisec->initialized = LABEL_INITIALIZED;
 
 	return 0;
@@ -5978,9 +6002,9 @@ static void selinux_inode_invalidate_secctx(struct inode *inode)
 {
 	struct inode_security_struct *isec = inode->i_security;
 
-	mutex_lock(&isec->lock);
+	spin_lock(&isec->lock);
 	isec->initialized = LABEL_INVALID;
-	mutex_unlock(&isec->lock);
+	spin_unlock(&isec->lock);
 }
 
 /*
diff --git a/security/selinux/include/objsec.h b/security/selinux/include/objsec.h
index c21e135..7e770e9 100644
--- a/security/selinux/include/objsec.h
+++ b/security/selinux/include/objsec.h
@@ -39,7 +39,8 @@ struct task_security_struct {
 
 enum label_initialized {
 	LABEL_INVALID,		/* invalid or not initialized */
-	LABEL_INITIALIZED	/* initialized */
+	LABEL_INITIALIZED,	/* initialized */
+	LABEL_PENDING
 };
 
 struct inode_security_struct {
@@ -52,7 +53,7 @@ struct inode_security_struct {
 	u32 sid;		/* SID of this object */
 	u16 sclass;		/* security class of this object */
 	unsigned char initialized;	/* initialization flag */
-	struct mutex lock;
+	struct spinlock lock;
 };
 
 struct file_security_struct {
-- 
2.7.4