linux-kernel - [PATCH v5 1/5] docs: proc: add documentation about mount restrictions

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Date:   Wed, 10 Mar 2021 19:19:56 +0100
From:   Alexey Gladkov <gladkov.alexey@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>
Cc:     Alexey Gladkov <legion@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: [PATCH v5 1/5] docs: proc: add documentation about mount restrictions

Signed-off-by: Alexey Gladkov <gladkov.alexey@...il.com>
---
 Documentation/filesystems/proc.rst | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 2fa69f710e2a..3daf0e7d1071 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -50,6 +50,7 @@ fixes/update part 1.1  Stefani Seibold <stefani@...bold.net>    June 9 2009
 
   4	Configuring procfs
   4.1	Mount options
+  4.2	Mount restrictions
 
   5	Filesystem behavior
 
@@ -2175,6 +2176,21 @@ information about processes information, just add identd to this group.
 subset=pid hides all top level files and directories in the procfs that
 are not related to tasks.
 
+4.2	Mount restrictions
+--------------------------
+
+The procfs can be mounted without any special restrictions if user namespace is
+not used. You only need to have permission to mount (CAP_SYS_ADMIN).
+
+If you are inside the user namespace, the kernel checks the instances of procfs
+available to you and will not allow procfs to be mounted if:
+
+  1. There is a bind mount of part of procfs visible. Whoever mounts should be
+     able to see the entire filesystem.
+  2. Mount is prohibited if a new mount overrides the readonly option or family
+     of atime options.
+  3. If any file or non-empty procfs directory is hidden by another filesystem.
+
 Chapter 5: Filesystem behavior
 ==============================
 
-- 
2.29.2