linux-kernel - [PATCH] cgroups: devices: add Documentation/ file

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Date:	Thu, 27 Mar 2008 10:12:27 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	lkml <linux-kernel@...r.kernel.org>
Subject: [PATCH] cgroups: devices: add Documentation/ file

Fill a Documentation/controllers/devices.txt file with a
modified text from the patch description.

Signed-off-by: Serge E. Hallyn <serue@...ibm.com>
---
 Documentation/controllers/devices.txt |   48 +++++++++++++++++++++++++++++++++
 1 files changed, 48 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/controllers/devices.txt

diff --git a/Documentation/controllers/devices.txt b/Documentation/controllers/devices.txt
new file mode 100644
index 0000000..a157f53
--- /dev/null
+++ b/Documentation/controllers/devices.txt
@@ -0,0 +1,48 @@
+Device Whitelist Controller
+
+1. Description:
+
+Implement a cgroup to track and enforce open and mknod restrictions
+on device files.  A device cgroup associates a device access
+whitelist with each cgroup.  A whitelist entry has 4 fields.
+'type' is a (all), c (char), or b (block).  'all' means it applies
+to all types and all major and minor numbers.  Major and minor are
+either an integer or * for all.  Access is a composition of r
+(read), w (write), and m (mknod).
+
+The root device cgroup starts with rwm to 'all'.  A child devices
+cgroup gets a copy of the parent.  Administrators can then remove
+devices from the whitelist or add new entries.  A child cgroup can
+never receive a device access which is denied its parent.  However
+when a device access is removed from a parent it will not also be
+removed from the child(ren).
+
+2. User Interface
+
+An entry is added using devices.allow, and removed using
+devices.deny.  For instance
+
+	echo 'c 1:3 mr' > /cgroups/1/devices.allow
+
+allows cgroup 1 to read and mknod the device usually known as
+/dev/null.  Doing
+
+	echo a > /cgroups/1/devices.deny
+
+will remove the default 'a *:* mrw' entry.
+
+3. Security
+
+Any task can move itself between cgroups.  This of clearly won't
+suffice, but we can decide the best way to adequately restrict
+movement as people get some experience with this.  We may just want
+to require CAP_SYS_ADMIN, which at least is a separate bit from
+CAP_MKNOD.  We may want to just refuse moving to a cgroup which
+isn't a descendent of the current one.  Or we may want to use
+CAP_MAC_ADMIN, since we really are trying to lock down root.
+
+CAP_SYS_ADMIN is needed to modify the whitelist or move another
+task to a new cgroup.  (Again we'll probably want to change that).
+
+A cgroup may not be granted more permissions than the cgroup's
+parent has.  
-- 
1.5.1

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