fuse: Remove user_ns check for FUSE_DEV_IOC_CLONE

Commit 8ed1f0e ("fs/fuse: fix ioctl type confusion") fixed a type
confusion bug by adding an ->f_op comparison.

Based on some off-list discussion back then, another check was added to
compare the f_cred->user_ns.  This is not for security reasons, but was
based on the idea that a FUSE device FD should be using the UID/GID
mappings of its f_cred->user_ns, and those translations are done using
fc->user_ns, which matches the f_cred->user_ns of the initial FUSE device
FD thanks to the check in fuse_fill_super().  See also commit 8cb0832
("fuse: Support fuse filesystems outside of init_user_ns").

But FUSE_DEV_IOC_CLONE is, at a higher level, a *cloning* operation that
copies an existing context (with a weird API that involves first opening
/dev/fuse, then tying the resulting new FUSE device FD to an existing FUSE
instance).  So if an application is already passing FUSE FDs across userns
boundaries and dealing with the resulting ID mapping complications somehow,
it doesn't make much sense to block this cloning operation.

I've heard that this check is an obstacle for some folks, and I don't see a
good reason to keep it, so remove it.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dev.c

@@ -2267,8 +2267,7 @@ static long fuse_dev_ioctl(struct file *file, unsigned int cmd,
 				 * Check against file->f_op because CUSE
 				 * uses the same ioctl handler.
 				 */
-				if (old->f_op == file->f_op &&
-				    old->f_cred->user_ns == file->f_cred->user_ns)
+				if (old->f_op == file->f_op)
 					fud = fuse_get_dev(old);
 
 				if (fud) {