1 files changed, 34 insertions, 6 deletions

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index f127625543b4..65da6e1b6de5 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -411,17 +411,45 @@ int fuse_do_getattr(struct inode *inode)
 	return err;
 }
 
+/*
+ * Calling into a user-controlled filesystem gives the filesystem
+ * daemon ptrace-like capabilities over the requester process.  This
+ * means, that the filesystem daemon is able to record the exact
+ * filesystem operations performed, and can also control the behavior
+ * of the requester process in otherwise impossible ways.  For example
+ * it can delay the operation for arbitrary length of time allowing
+ * DoS against the requester.
+ *
+ * For this reason only those processes can call into the filesystem,
+ * for which the owner of the mount has ptrace privilege.  This
+ * excludes processes started by other users, suid or sgid processes.
+ */
+static int fuse_allow_task(struct fuse_conn *fc, struct task_struct *task)
+{
+	if (fc->flags & FUSE_ALLOW_OTHER)
+		return 1;
+
+	if (task->euid == fc->user_id &&
+	    task->suid == fc->user_id &&
+	    task->uid == fc->user_id &&
+	    task->egid == fc->group_id &&
+	    task->sgid == fc->group_id &&
+	    task->gid == fc->group_id)
+		return 1;
+
+	return 0;
+}
+
 static int fuse_revalidate(struct dentry *entry)
 {
 	struct inode *inode = entry->d_inode;
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_conn *fc = get_fuse_conn(inode);
 
-	if (get_node_id(inode) == FUSE_ROOT_ID) {
-		if (!(fc->flags & FUSE_ALLOW_OTHER) &&
-		    current->fsuid != fc->user_id)
-			return -EACCES;
-	} else if (time_before_eq(jiffies, fi->i_time))
+	if (!fuse_allow_task(fc, current))
+		return -EACCES;
+	if (get_node_id(inode) != FUSE_ROOT_ID &&
+	    time_before_eq(jiffies, fi->i_time))
 		return 0;
 
 	return fuse_do_getattr(inode);
@@ -431,7 +459,7 @@ static int fuse_permission(struct inode *inode, int mask, struct nameidata *nd)
 {
 	struct fuse_conn *fc = get_fuse_conn(inode);
 
-	if (!(fc->flags & FUSE_ALLOW_OTHER) && current->fsuid != fc->user_id)
+	if (!fuse_allow_task(fc, current))
 		return -EACCES;
 	else if (fc->flags & FUSE_DEFAULT_PERMISSIONS) {
 		int err = generic_permission(inode, mask, NULL);