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);