[RFC 5/5 v4] procfs: add documentation for procfs mount options

[Vasiliy Kulikov]

Add documentation for procfs mount options.

v4 - grammar fixes.

Signed-off-by: Vasiliy Kulikov <segoon@xxxxxxxxxxxx>
---
 Documentation/filesystems/proc.txt |   51 ++++++++++++++++++++++++++++++++++++
 1 files changed, 51 insertions(+), 0 deletions(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index f481780..327a640 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -41,6 +41,8 @@ Table of Contents
   3.5	/proc/<pid>/mountinfo - Information about mounts
   3.6	/proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
 
+  4	Configuring procfs
+  4.1	Mount options
 
 ------------------------------------------------------------------------------
 Preface
@@ -1541,3 +1543,52 @@ a task to set its own or one of its thread siblings comm value. The comm value
 is limited in size compared to the cmdline value, so writing anything longer
 then the kernel's TASK_COMM_LEN (currently 16 chars) will result in a truncated
 comm value.
+
+
+------------------------------------------------------------------------------
+Configuring procfs
+------------------------------------------------------------------------------
+
+4.1	Mount options
+---------------------
+
+The following mount options are supported:
+
+	hidepid=	Set /proc/<pid>/ access mode.
+	hidenet		Hide /proc/<pid>/net/ from nonauthorized users.
+	nohidenet	Don't hide /proc/<pid>/net/ from nonauthorized users.
+	gid=		Set the group authorized to learn processes and
+			networking information.
+
+hidepid=0 means classic mode - everybody may access all /proc/<pid>/ directories
+(default).
+
+hidepid=1 means users may not access any /proc/<pid>/ directories but their
+own.  Sensitive files like cmdline, io, sched*, status, wchan are now protected
+against other users.  This makes it impossible to learn whether any user runs
+specific program (given the program doesn't reveal itself by its behaviour).
+As an additional bonus, as /proc/<pid>/cmdline is unaccessible for other users,
+poorly written programs passing sensitive information via program arguments are
+now protected against local eavesdroppers.
+
+hidepid=2 means hidepid=1 plus all /proc/<pid>/ will be fully invisible to other
+users.  It doesn't mean that it hides whether a process with a specific
+pid value exists (it can be learned by other means, e.g. by sending signals),
+but it hides process' uid and gid, which may be learned by stat()'ing
+/proc/<pid>/ otherwise.  It greatly complicates an intruder's task of gathering
+information about running processes, whether some daemon runs with elevated privileges,
+whether another user runs some sensitive program, whether other users run any
+program at all, etc.
+
+hidenet means /proc/<pid>/net/ will be accessible to processes with
+CAP_NET_ADMIN capability or to members of a special group.  It means
+nonauthorized users may not learn any networking connections information.  If
+network namespaces support is enabled (CONFIG_NET_NS=y) then common users would
+obtain net directory, but all files would indicate no networking activity at
+all.  If network namespaces are disabled, net directory is unaccessible to
+common users.
+
+gid= means group authorized to learn processes information prohibited by
+hidepid= and networking information prohibited by hidenet.  If you use some
+daemon like identd which has to learn information about net/processes
+information, just add identd to this group.
-- 
1.7.0.4

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