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Message-ID: <20180223144807.1180-8-igor.stoppa@huawei.com>
Date: Fri, 23 Feb 2018 16:48:07 +0200
From: Igor Stoppa <igor.stoppa@...wei.com>
To: <david@...morbit.com>, <willy@...radead.org>, <keescook@...omium.org>,
<mhocko@...nel.org>
CC: <labbott@...hat.com>, <linux-security-module@...r.kernel.org>,
<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
<kernel-hardening@...ts.openwall.com>, Igor Stoppa <igor.stoppa@...wei.com>
Subject: [PATCH 7/7] Documentation for Pmalloc
Detailed documentation about the protectable memory allocator.
Signed-off-by: Igor Stoppa <igor.stoppa@...wei.com>
---
Documentation/core-api/index.rst | 1 +
Documentation/core-api/pmalloc.rst | 114 +++++++++++++++++++++++++++++++++++++
2 files changed, 115 insertions(+)
create mode 100644 Documentation/core-api/pmalloc.rst
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index c670a8031786..8f5de42d6571 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -25,6 +25,7 @@ Core utilities
genalloc
errseq
printk-formats
+ pmalloc
Interfaces for kernel debugging
===============================
diff --git a/Documentation/core-api/pmalloc.rst b/Documentation/core-api/pmalloc.rst
new file mode 100644
index 000000000000..d9725870444e
--- /dev/null
+++ b/Documentation/core-api/pmalloc.rst
@@ -0,0 +1,114 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Protectable memory allocator
+============================
+
+Purpose
+-------
+
+The pmalloc library is meant to provide R/O status to data that, for some
+reason, could neither be declared as constant, nor could it take advantage
+of the qualifier __ro_after_init, but is write-once and read-only in spirit.
+It protects data from both accidental and malicious overwrites.
+
+Example: A policy that is loaded from userspace.
+
+
+Concept
+-------
+
+pmalloc builds on top of genalloc, using the same concept of memory pools.
+
+The value added by pmalloc is that now the memory contained in a pool can
+become R/O, for the rest of the life of the pool.
+
+Different kernel drivers and threads can use different pools, for finer
+control of what becomes R/O and when. And for improved lockless concurrency.
+
+
+Caveats
+-------
+
+- Memory freed while a pool is not yet protected will be reused.
+
+- Once a pool is protected, it's not possible to allocate any more memory
+ from it.
+
+- Memory "freed" from a protected pool indicates that such memory is not
+ in use anymore by the requester; however, it will not become available
+ for further use, until the pool is destroyed.
+
+- Before destroying a pool, all the memory allocated from it must be
+ released.
+
+- pmalloc does not provide locking support with respect to allocating vs
+ protecting an individual pool, for performance reasons.
+ It is recommended not to share the same pool between unrelated functions.
+ Should sharing be a necessity, the user of the shared pool is expected
+ to implement locking for that pool.
+
+- pmalloc uses genalloc to optimize the use of the space it allocates
+ through vmalloc. Some more TLB entries will be used, however less than
+ in the case of using vmalloc directly. The exact number depends on the
+ size of each allocation request and possible slack.
+
+- Considering that not much data is supposed to be dynamically allocated
+ and then marked as read-only, it shouldn't be an issue that the address
+ range for pmalloc is limited, on 32-bit systems.
+
+- Regarding SMP systems, the allocations are expected to happen mostly
+ during an initial transient, after which there should be no more need to
+ perform cross-processor synchronizations of page tables.
+
+- To facilitate the conversion of existing code to pmalloc pools, several
+ helper functions are provided, mirroring their kmalloc counterparts.
+
+
+Use
+---
+
+The typical sequence, when using pmalloc, is:
+
+1. create a pool
+
+.. kernel-doc:: include/linux/pmalloc.h
+ :functions: pmalloc_create_pool
+
+2. [optional] pre-allocate some memory in the pool
+
+.. kernel-doc:: include/linux/pmalloc.h
+ :functions: pmalloc_prealloc
+
+3. issue one or more allocation requests to the pool with locking as needed
+
+.. kernel-doc:: include/linux/pmalloc.h
+ :functions: pmalloc
+
+.. kernel-doc:: include/linux/pmalloc.h
+ :functions: pzalloc
+
+4. initialize the memory obtained with desired values
+
+5. [optional] iterate over points 3 & 4 as needed
+
+6. write-protect the pool
+
+.. kernel-doc:: include/linux/pmalloc.h
+ :functions: pmalloc_protect_pool
+
+7. use in read-only mode the handles obtained through the allocations
+
+8. [optional] release all the memory allocated
+
+.. kernel-doc:: include/linux/pmalloc.h
+ :functions: pfree
+
+9. [optional, but depends on point 8] destroy the pool
+
+.. kernel-doc:: include/linux/pmalloc.h
+ :functions: pmalloc_destroy_pool
+
+API
+---
+
+.. kernel-doc:: include/linux/pmalloc.h
--
2.14.1
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