linux-kernel - [PATCH v2] intelrdt: resctrl: recommend locking for resctrlfs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20161201145541.GA19232@amt.cnet>
Date:   Thu, 1 Dec 2016 12:55:44 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: [PATCH v2] intelrdt: resctrl: recommend locking for resctrlfs 


There is a locking problem between different applications
reading/writing to resctrlfs directory at the same time (read the patch
below for details).

Suggest a standard locking scheme for applications to use.

Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>

---
v2: Improve commentary about generality of locking problem ("Yu, Fenghua")
    Proper patch -p level (Thomas Gleixner)

diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt
index d918d26..14b3214 100644
--- a/Documentation/x86/intel_rdt_ui.txt
+++ b/Documentation/x86/intel_rdt_ui.txt
@@ -212,3 +212,33 @@ Finally we move core 4-7 over to the new group and make sure that the
 kernel and the tasks running there get 50% of the cache.
 
 # echo C0 > p0/cpus
+
+4) Locking between applications
+
+Certain operations on the resctrl filesystem, composed of
+read / writes to multiple files, must be atomic.
+
+As an example, the allocation of an exclusive reservation
+of L3 cache involves:
+
+        1. read list of cbmmasks for each directory
+        2. find a contiguous set of bits in the global CBM bitmask
+           that is clear in any of the directory cbmmasks
+        3. create a new directory
+        4. set the bits found in step 2 to the new directory "schemata"
+           file
+
+If two applications attempting to allocate space race with each other
+(if two processes execute the steps above in a interlocked fashion),
+they can end up using the same bits of CBMMASK, which renders the
+reservations non-exclusive but shared.
+
+To coordinate atomic operations on resctrl and avoid the problem
+above, the following locking procedure is recommended:
+
+A) open /var/lock/resctrl/fs.lock with O_CREAT|O_EXCL.
+B) if success, write pid of program accessing the directory
+   structure to this file.
+C) read/write the directory structure.
+D) remove file.
+