fuse: Add module param for CAP_SYS_ADMIN access bypassing allow_other

Since commit 73f03c2b4b52 ("fuse: Restrict allow_other to the superblock's
namespace or a descendant"), access to allow_other FUSE filesystems has
been limited to users in the mounting user namespace or descendants. This
prevents a process that is privileged in its userns - but not its parent
namespaces - from mounting a FUSE fs w/ allow_other that is accessible to
processes in parent namespaces.

While this restriction makes sense overall it breaks a legitimate usecase:
I have a tracing daemon which needs to peek into process' open files in
order to symbolicate - similar to 'perf'. The daemon is a privileged
process in the root userns, but is unable to peek into FUSE filesystems
mounted by processes in child namespaces.

This patch adds a module param, allow_sys_admin_access, to act as an escape
hatch for this descendant userns logic and for the allow_other mount option
in general. Setting allow_sys_admin_access allows processes with
CAP_SYS_ADMIN in the initial userns to access FUSE filesystems irrespective
of the mounting userns or whether allow_other was set. A sysadmin setting
this param must trust FUSEs on the host to not DoS processes as described
in 73f03c2b4b52.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fuse: update project homepage

As stated in https://sourceforge.net/projects/fuse/, "the FUSE project has
moved to https://github.com/libfuse/" in 22-Dec-2015. Update URLs to
reflect this.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

docs: filesystems: fuse.rst: supress a Sphinx warning

Get rid of this warning:

Documentation/filesystems/fuse.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.

Fixes: 8ab13bca428b ("Documentation: filesystems: convert fuse to RST")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/cad541ec7d8d220d57bd5d097d60c62da64054ac.1583250595.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation: filesystems: convert fuse to RST

Converts fuse.txt to reStructuredText format, improving the presentation
without changing much of the underlying content.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>