From: Igor Stoppa <igor.stoppa@gmail.com>
To: Mimi Zohar <zohar@linux.vnet.ibm.com>,
Kees Cook <keescook@chromium.org>,
Matthew Wilcox <willy@infradead.org>,
Dave Chinner <david@fromorbit.com>,
James Morris <jmorris@namei.org>,
Michal Hocko <mhocko@kernel.org>,
kernel-hardening@lists.openwall.com,
linux-integrity@vger.kernel.org,
linux-security-module@vger.kernel.org
Cc: igor.stoppa@huawei.com, Dave Hansen <dave.hansen@linux.intel.com>,
Jonathan Corbet <corbet@lwn.net>,
Laura Abbott <labbott@redhat.com>,
Randy Dunlap <rdunlap@infradead.org>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 10/17] prmem: documentation
Date: Wed, 24 Oct 2018 00:34:57 +0300 [thread overview]
Message-ID: <20181023213504.28905-11-igor.stoppa@huawei.com> (raw)
In-Reply-To: <20181023213504.28905-1-igor.stoppa@huawei.com>
Documentation for protected memory.
Topics covered:
* static memory allocation
* dynamic memory allocation
* write-rare
Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>
CC: Jonathan Corbet <corbet@lwn.net>
CC: Randy Dunlap <rdunlap@infradead.org>
CC: Mike Rapoport <rppt@linux.vnet.ibm.com>
CC: linux-doc@vger.kernel.org
CC: linux-kernel@vger.kernel.org
---
Documentation/core-api/index.rst | 1 +
Documentation/core-api/prmem.rst | 172 +++++++++++++++++++++++++++++++
MAINTAINERS | 1 +
3 files changed, 174 insertions(+)
create mode 100644 Documentation/core-api/prmem.rst
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 26b735cefb93..1a90fa878d8d 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -31,6 +31,7 @@ Core utilities
gfp_mask-from-fs-io
timekeeping
boot-time-mm
+ prmem
Interfaces for kernel debugging
===============================
diff --git a/Documentation/core-api/prmem.rst b/Documentation/core-api/prmem.rst
new file mode 100644
index 000000000000..16d7edfe327a
--- /dev/null
+++ b/Documentation/core-api/prmem.rst
@@ -0,0 +1,172 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _prmem:
+
+Memory Protection
+=================
+
+:Date: October 2018
+:Author: Igor Stoppa <igor.stoppa@huawei.com>
+
+Foreword
+--------
+- In a typical system using some sort of RAM as execution environment,
+ **all** memory is initially writable.
+
+- It must be initialized with the appropriate content, be it code or data.
+
+- Said content typically undergoes modifications, i.e. relocations or
+ relocation-induced changes.
+
+- The present document doesn't address such transient.
+
+- Kernel code is protected at system level and, unlike data, it doesn't
+ require special attention.
+
+Protection mechanism
+--------------------
+
+- When available, the MMU can write protect memory pages that would be
+ otherwise writable.
+
+- The protection has page-level granularity.
+
+- An attempt to overwrite a protected page will trigger an exception.
+- **Write protected data must go exclusively to write protected pages**
+- **Writable data must go exclusively to writable pages**
+
+Available protections for kernel data
+-------------------------------------
+
+- **constant**
+ Labelled as **const**, the data is never supposed to be altered.
+ It is statically allocated - if it has any memory footprint at all.
+ The compiler can even optimize it away, where possible, by replacing
+ references to a **const** with its actual value.
+
+- **read only after init**
+ By tagging an otherwise ordinary statically allocated variable with
+ **__ro_after_init**, it is placed in a special segment that will
+ become write protected, at the end of the kernel init phase.
+ The compiler has no notion of this restriction and it will treat any
+ write operation on such variable as legal. However, assignments that
+ are attempted after the write protection is in place, will cause
+ exceptions.
+
+- **write rare after init**
+ This can be seen as variant of read only after init, which uses the
+ tag **__wr_after_init**. It is also limited to statically allocated
+ memory. It is still possible to alter this type of variables, after
+ the kernel init phase is complete, however it can be done exclusively
+ with special functions, instead of the assignment operator. Using the
+ assignment operator after conclusion of the init phase will still
+ trigger an exception. It is not possible to transition a certain
+ variable from __wr_ater_init to a permanent read-only status, at
+ runtime.
+
+- **dynamically allocated write-rare / read-only**
+ After defining a pool, memory can be obtained through it, primarily
+ through the **pmalloc()** allocator. The exact writability state of the
+ memory obtained from **pmalloc()** and friends can be configured when
+ creating the pool. At any point it is possible to transition to a less
+ permissive write status the memory currently associated to the pool.
+ Once memory has become read-only, it the only valid operation, beside
+ reading, is to released it, by destroying the pool it belongs to.
+
+
+Protecting dynamically allocated memory
+---------------------------------------
+
+When dealing with dynamically allocated memory, three options are
+ available for configuring its writability state:
+
+- **Options selected when creating a pool**
+ When creating the pool, it is possible to choose one of the following:
+ - **PMALLOC_MODE_RO**
+ - Writability at allocation time: *WRITABLE*
+ - Writability at protection time: *NONE*
+ - **PMALLOC_MODE_WR**
+ - Writability at allocation time: *WRITABLE*
+ - Writability at protection time: *WRITE-RARE*
+ - **PMALLOC_MODE_AUTO_RO**
+ - Writability at allocation time:
+ - the latest allocation: *WRITABLE*
+ - every other allocation: *NONE*
+ - Writability at protection time: *NONE*
+ - **PMALLOC_MODE_AUTO_WR**
+ - Writability at allocation time:
+ - the latest allocation: *WRITABLE*
+ - every other allocation: *WRITE-RARE*
+ - Writability at protection time: *WRITE-RARE*
+ - **PMALLOC_MODE_START_WR**
+ - Writability at allocation time: *WRITE-RARE*
+ - Writability at protection time: *WRITE-RARE*
+
+ **Remarks:**
+ - The "AUTO" modes perform automatic protection of the content, whenever
+ the current vmap_area is used up and a new one is allocated.
+ - At that point, the vmap_area being phased out is protected.
+ - The size of the vmap_area depends on various parameters.
+ - It might not be possible to know for sure *when* certain data will
+ be protected.
+ - The functionality is provided as tradeoff between hardening and speed.
+ - Its usefulness depends on the specific use case at hand
+ - The "START_WR" mode is the only one which provides immediate protection, at the cost of speed.
+
+- **Protecting the pool**
+ This is achieved with **pmalloc_protect_pool()**
+ - Any vmap_area currently in the pool is write-protected according to its initial configuration.
+ - Any residual space still available from the current vmap_area is lost, as the area is protected.
+ - **protecting a pool after every allocation will likely be very wasteful**
+ - Using PMALLOC_MODE_START_WR is likely a better choice.
+
+- **Upgrading the protection level**
+ This is achieved with **pmalloc_make_pool_ro()**
+ - it turns the present content of a write-rare pool into read-only
+ - can be useful when the content of the memory has settled
+
+
+Caveats
+-------
+- Freeing of memory is not supported. Pages will be returned to the
+ system upon destruction of their memory pool.
+
+- The address range available for vmalloc (and thus for pmalloc too) is
+ limited, on 32-bit systems. However it shouldn't be an issue, since not
+ much data is expected to be dynamically allocated and turned into
+ write-protected.
+
+- Regarding SMP systems, changing state of pages and altering mappings
+ requires performing cross-processor synchronizations of page tables.
+ This is an additional reason for limiting the use of write rare.
+
+- Not only the pmalloc memory must be protected, but also any reference to
+ it that might become the target for an attack. The attack would replace
+ a reference to the protected memory with a reference to some other,
+ unprotected, memory.
+
+- The users of rare write must take care of ensuring the atomicity of the
+ action, respect to the way they use the data being altered; for example,
+ take a lock before making a copy of the value to modify (if it's
+ relevant), then alter it, issue the call to rare write and finally
+ release the lock. Some special scenario might be exempt from the need
+ for locking, but in general rare-write must be treated as an operation
+ that can incur into races.
+
+- pmalloc relies on virtual memory areas and will therefore use more
+ tlb entries. It still does a better job of it, compared to invoking
+ vmalloc for each allocation, but it is undeniably less optimized wrt to
+ TLB use than using the physmap directly, through kmalloc or similar.
+
+
+Utilization
+-----------
+
+**add examples here**
+
+API
+---
+
+.. kernel-doc:: include/linux/prmem.h
+.. kernel-doc:: mm/prmem.c
+.. kernel-doc:: include/linux/prmemextra.h
diff --git a/MAINTAINERS b/MAINTAINERS
index ea979a5a9ec9..246b1a1cc8bb 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9463,6 +9463,7 @@ F: include/linux/prmemextra.h
F: mm/prmem.c
F: mm/test_write_rare.c
F: mm/test_pmalloc.c
+F: Documentation/core-api/prmem.rst
MEMORY MANAGEMENT
L: linux-mm@kvack.org
--
2.17.1
next prev parent reply other threads:[~2018-10-23 21:37 UTC|newest]
Thread overview: 137+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20181023213504.28905-1-igor.stoppa@huawei.com>
2018-10-23 21:34 ` [PATCH 01/17] prmem: linker section for static write rare Igor Stoppa
2018-10-23 21:34 ` [PATCH 02/17] prmem: write rare for static allocation Igor Stoppa
2018-10-25 0:24 ` Dave Hansen
2018-10-29 18:03 ` Igor Stoppa
2018-10-26 9:41 ` Peter Zijlstra
2018-10-29 20:01 ` Igor Stoppa
2018-10-23 21:34 ` [PATCH 03/17] prmem: vmalloc support for dynamic allocation Igor Stoppa
2018-10-25 0:26 ` Dave Hansen
2018-10-29 18:07 ` Igor Stoppa
2018-10-23 21:34 ` [PATCH 04/17] prmem: " Igor Stoppa
2018-10-23 21:34 ` [PATCH 05/17] prmem: shorthands for write rare on common types Igor Stoppa
2018-10-25 0:28 ` Dave Hansen
2018-10-29 18:12 ` Igor Stoppa
2018-10-23 21:34 ` [PATCH 06/17] prmem: test cases for memory protection Igor Stoppa
2018-10-24 3:27 ` Randy Dunlap
2018-10-24 14:24 ` Igor Stoppa
2018-10-25 16:43 ` Dave Hansen
2018-10-29 18:16 ` Igor Stoppa
2018-10-23 21:34 ` [PATCH 07/17] prmem: lkdtm tests " Igor Stoppa
2018-10-23 21:34 ` [PATCH 08/17] prmem: struct page: track vmap_area Igor Stoppa
2018-10-24 3:12 ` Matthew Wilcox
2018-10-24 23:01 ` Igor Stoppa
2018-10-25 2:13 ` Matthew Wilcox
2018-10-29 18:21 ` Igor Stoppa
2018-10-23 21:34 ` [PATCH 09/17] prmem: hardened usercopy Igor Stoppa
2018-10-29 11:45 ` Chris von Recklinghausen
2018-10-29 18:24 ` Igor Stoppa
2018-10-23 21:34 ` Igor Stoppa [this message]
2018-10-24 3:48 ` [PATCH 10/17] prmem: documentation Randy Dunlap
2018-10-24 14:30 ` Igor Stoppa
2018-10-24 23:04 ` Mike Rapoport
2018-10-29 19:05 ` Igor Stoppa
2018-10-26 9:26 ` Peter Zijlstra
2018-10-26 10:20 ` Matthew Wilcox
2018-10-29 19:28 ` Igor Stoppa
2018-10-26 10:46 ` Kees Cook
2018-10-28 18:31 ` Peter Zijlstra
2018-10-29 21:04 ` Igor Stoppa
2018-10-30 15:26 ` Peter Zijlstra
2018-10-30 16:37 ` Kees Cook
2018-10-30 17:06 ` Andy Lutomirski
2018-10-30 17:58 ` Matthew Wilcox
2018-10-30 18:03 ` Dave Hansen
2018-10-31 9:18 ` Peter Zijlstra
2018-10-30 18:28 ` Tycho Andersen
2018-10-30 19:20 ` Matthew Wilcox
2018-10-30 20:43 ` Igor Stoppa
2018-10-30 21:02 ` Andy Lutomirski
2018-10-30 21:07 ` Kees Cook
2018-10-30 21:25 ` Igor Stoppa
2018-10-30 22:15 ` Igor Stoppa
2018-10-31 10:11 ` Peter Zijlstra
2018-10-31 20:38 ` Andy Lutomirski
2018-10-31 20:53 ` Andy Lutomirski
2018-10-31 9:45 ` Peter Zijlstra
2018-10-30 21:35 ` Matthew Wilcox
2018-10-30 21:49 ` Igor Stoppa
2018-10-31 4:41 ` Andy Lutomirski
2018-10-31 9:08 ` Igor Stoppa
2018-10-31 19:38 ` Igor Stoppa
2018-10-31 10:02 ` Peter Zijlstra
2018-10-31 20:36 ` Andy Lutomirski
2018-10-31 21:00 ` Peter Zijlstra
2018-10-31 22:57 ` Andy Lutomirski
2018-10-31 23:10 ` Igor Stoppa
2018-10-31 23:19 ` Andy Lutomirski
2018-10-31 23:26 ` Igor Stoppa
2018-11-01 8:21 ` Thomas Gleixner
2018-11-01 15:58 ` Igor Stoppa
2018-11-01 17:08 ` Peter Zijlstra
2018-10-30 18:51 ` Andy Lutomirski
2018-10-30 19:14 ` Kees Cook
2018-10-30 21:25 ` Matthew Wilcox
2018-10-30 21:55 ` Igor Stoppa
2018-10-30 22:08 ` Matthew Wilcox
2018-10-31 9:29 ` Peter Zijlstra
2018-10-30 23:18 ` Nadav Amit
2018-10-31 9:08 ` Peter Zijlstra
2018-11-01 16:31 ` Nadav Amit
2018-11-02 21:11 ` Nadav Amit
2018-10-31 9:36 ` Peter Zijlstra
2018-10-31 11:33 ` Matthew Wilcox
2018-11-13 14:25 ` Igor Stoppa
2018-11-13 17:16 ` Andy Lutomirski
2018-11-13 17:43 ` Nadav Amit
2018-11-13 17:47 ` Andy Lutomirski
2018-11-13 18:06 ` Nadav Amit
2018-11-13 18:31 ` Igor Stoppa
2018-11-13 18:33 ` Igor Stoppa
2018-11-13 18:36 ` Andy Lutomirski
2018-11-13 19:03 ` Igor Stoppa
2018-11-21 16:34 ` Igor Stoppa
2018-11-21 17:36 ` Nadav Amit
2018-11-21 18:01 ` Igor Stoppa
2018-11-21 18:15 ` Andy Lutomirski
2018-11-22 19:27 ` Igor Stoppa
2018-11-22 20:04 ` Matthew Wilcox
2018-11-22 20:53 ` Andy Lutomirski
2018-12-04 12:34 ` Igor Stoppa
2018-11-13 18:48 ` Andy Lutomirski
2018-11-13 19:35 ` Igor Stoppa
2018-11-13 18:26 ` Igor Stoppa
2018-11-13 18:35 ` Andy Lutomirski
2018-11-13 19:01 ` Igor Stoppa
2018-10-31 9:27 ` Igor Stoppa
2018-10-26 11:09 ` Markus Heiser
2018-10-29 19:35 ` Igor Stoppa
2018-10-26 15:05 ` Jonathan Corbet
2018-10-29 19:38 ` Igor Stoppa
2018-10-29 20:35 ` Igor Stoppa
2018-10-23 21:34 ` [PATCH 11/17] prmem: llist: use designated initializer Igor Stoppa
2018-10-23 21:34 ` [PATCH 12/17] prmem: linked list: set alignment Igor Stoppa
2018-10-26 9:31 ` Peter Zijlstra
2018-10-23 21:35 ` [PATCH 13/17] prmem: linked list: disable layout randomization Igor Stoppa
2018-10-24 13:43 ` Alexey Dobriyan
2018-10-29 19:40 ` Igor Stoppa
2018-10-26 9:32 ` Peter Zijlstra
2018-10-26 10:17 ` Matthew Wilcox
2018-10-30 15:39 ` Peter Zijlstra
2018-10-23 21:35 ` [PATCH 14/17] prmem: llist, hlist, both plain and rcu Igor Stoppa
2018-10-24 11:37 ` Mathieu Desnoyers
2018-10-24 14:03 ` Igor Stoppa
2018-10-24 14:56 ` Tycho Andersen
2018-10-24 22:52 ` Igor Stoppa
2018-10-25 8:11 ` Tycho Andersen
2018-10-28 9:52 ` Steven Rostedt
2018-10-29 19:43 ` Igor Stoppa
2018-10-26 9:38 ` Peter Zijlstra
2018-10-23 21:35 ` [PATCH 15/17] prmem: test cases for prlist and prhlist Igor Stoppa
2018-10-23 21:35 ` [PATCH 16/17] prmem: pratomic-long Igor Stoppa
2018-10-25 0:13 ` Peter Zijlstra
2018-10-29 21:17 ` Igor Stoppa
2018-10-30 15:58 ` Peter Zijlstra
2018-10-30 16:28 ` Will Deacon
2018-10-31 9:10 ` Peter Zijlstra
2018-11-01 3:28 ` Kees Cook
2018-10-23 21:35 ` [PATCH 17/17] prmem: ima: turn the measurements list write rare Igor Stoppa
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