Patch "fuse: fix clearing suid, sgid for chown()" 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 clearing suid, sgid for chown()

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-clearing-suid-sgid-for-chown.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: Tue, 6 Dec 2016 16:18:45 +0100
Subject: fuse: fix clearing suid, sgid for chown()

From: Miklos Szeredi <[email protected]>

commit c01638f5d919728f565bf8b5e0a6a159642df0d9 upstream.

Basically, the pjdfstests set the ownership of a file to 06555, and then chowns it (as root) to a new uid/gid. Prior to commit a09f99eddef4 ("fuse:
fix killing s[ug]id in setattr"), fuse would send down a setattr with both the uid/gid change and a new mode. Now, it just sends down the uid/gid change.

Technically this is NOTABUG, since POSIX doesn't _require_ that we clear these bits for a privileged process, but Linux (wisely) has done that and I think we don't want to change that behavior here.

This is caused by the use of should_remove_suid(), which will always return 0 when the process has CAP_FSETID.

In fact we really don't need to be calling should_remove_suid() at all, since we've already been indicated that we should remove the suid, we just don't want to use a (very) stale mode for that.

This patch should fix the above as well as simplify the logic.

---
 fs/fuse/dir.c |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

@@ -1709,8 +1709,6 @@ static int fuse_setattr(struct dentry *e
 		return -EACCES;
 
 	if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) {
-		int kill;
-
 		attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID |
 				    ATTR_MODE);
 		/*

@@ -1722,12 +1720,11 @@ static int fuse_setattr(struct dentry *e
 			return ret;
 
 		attr->ia_mode = inode->i_mode;
-		kill = should_remove_suid(entry);
-		if (kill & ATTR_KILL_SUID) {
+		if (inode->i_mode & S_ISUID) {
 			attr->ia_valid |= ATTR_MODE;
 			attr->ia_mode &= ~S_ISUID;
 		}
-		if (kill & ATTR_KILL_SGID) {
+		if ((inode->i_mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
 			attr->ia_valid |= ATTR_MODE;
 			attr->ia_mode &= ~S_ISGID;
 		}

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

queue-4.8/fuse-fix-clearing-suid-sgid-for-chown.patch