Date: Sun, 11 Feb 2018 05:19:20 +0200 From: Igor Stoppa <igor.stoppa@...wei.com> To: <willy@...radead.org>, <rdunlap@...radead.org>, <corbet@....net>, <keescook@...omium.org>, <mhocko@...nel.org>, <labbott@...hat.com>, <jglisse@...hat.com>, <hch@...radead.org> CC: <cl@...ux.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 6/6] 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..1afc13b6a74d --- /dev/null +++ b/Documentation/core-api/pmalloc.rst @@ -0,0 +1,114 @@ +SPDX-License-Identifier: CC-BY-SA-4.0 + +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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