Patch "fuse: fix killing s[ug]id in setattr" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

fuse: fix killing s[ug]id in setattr

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
fuse-fix-killing-sid-in-setattr.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Miklos Szeredi <[email protected]>
Date: Sat, 1 Oct 2016 07:32:32 +0200
Subject: fuse: fix killing s[ug]id in setattr

From: Miklos Szeredi <[email protected]>

commit a09f99eddef44035ec764075a37bace8181bec38 upstream.

Fuse allowed VFS to set mode in setattr in order to clear suid/sgid on chown and truncate, and (since writeback_cache) write. The problem with this is that it'll potentially restore a stale mode.

The poper fix would be to let the filesystems do the suid/sgid clearing on the relevant operations. Possibly some are already doing it but there's no way we can detect this.

So fix this by refreshing and recalculating the mode. Do this only if ATTR_KILL_S[UG]ID is set to not destroy performance for writes. This is still racy but the size of the window is reduced.

---
 fs/fuse/dir.c |   32 ++++++++++++++++++++++++++++----
 1 file changed, 28 insertions(+), 4 deletions(-)

@@ -1702,16 +1702,40 @@ error:
 static int fuse_setattr(struct dentry *entry, struct iattr *attr)
 {
 	struct inode *inode = d_inode(entry);
+	struct file *file = (attr->ia_valid & ATTR_FILE) ? attr->ia_file : NULL;
 	int ret;
 
 	if (!fuse_allow_current_process(get_fuse_conn(inode)))
 		return -EACCES;
 
-	if (attr->ia_valid & ATTR_FILE)
-		ret = fuse_do_setattr(inode, attr, attr->ia_file);
-	else
-		ret = fuse_do_setattr(inode, attr, NULL);
+	if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) {
+		int kill;
 
+		attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID |
+				    ATTR_MODE);
+		/*
+		 * ia_mode calculation may have used stale i_mode.  Refresh and
+		 * recalculate.
+		 */
+		ret = fuse_do_getattr(inode, NULL, file);
+		if (ret)
+			return ret;
+
+		attr->ia_mode = inode->i_mode;
+		kill = should_remove_suid(entry);
+		if (kill & ATTR_KILL_SUID) {
+			attr->ia_valid |= ATTR_MODE;
+			attr->ia_mode &= ~S_ISUID;
+		}
+		if (kill & ATTR_KILL_SGID) {
+			attr->ia_valid |= ATTR_MODE;
+			attr->ia_mode &= ~S_ISGID;
+		}
+	}
+	if (!attr->ia_valid)
+		return 0;
+
+	ret = fuse_do_setattr(inode, attr, file);
 	if (!ret) {
 		/* Directory mode changed, may need to revalidate access */
 		if (d_is_dir(entry) && (attr->ia_valid & ATTR_MODE))

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/fuse-listxattr-verify-xattr-list.patch queue-4.8/fuse-invalidate-dir-dentry-after-chmod.patch queue-4.8/fuse-fix-killing-sid-in-setattr.patch