Date: Tue, 27 Mar 2018 18:37:42 +0300 From: Igor Stoppa <igor.stoppa@...wei.com> To: <willy@...radead.org>, <keescook@...omium.org>, <mhocko@...nel.org> CC: <david@...morbit.com>, <rppt@...ux.vnet.ibm.com>, <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@...il.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 | 107 +++++++++++++++++++++++++++++++++++++ 2 files changed, 108 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..c14907485137 --- /dev/null +++ b/Documentation/core-api/pmalloc.rst @@ -0,0 +1,107 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _pmalloc: + +Protectable memory allocator +============================ + +Purpose +------- + +The pmalloc library is meant to provide read-only 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. At least as long as it doesn't get teared down. +It protects data from both accidental and malicious overwrites. + +Example: A policy that is loaded from userspace. + + +Concept +------- + +The MMU available in the system can be used to write protect memory pages. +Unfortunately this feature cannot be used as-it-is, to protect sensitive +data, because this potentially read-only data is typically interleaved +with other data, which must stay writeable. + +pmalloc introduces the concept of protectable memory pools. +A pool contains a list of areas of virtually contiguous pages of +memory. An area is the minimum amount of memory that pmalloc allows to +protect, because the user might have allocated a memory range that +crosses the boundary between pages. + +When an allocation is performed, if there is not enough memory already +available in the pool, a new area of suitable size is grabbed. +The size chosen is the largest between the roundup (to PAGE_SIZE) of +the request from pmalloc and friends and the refill parameter specified +when creating the pool. + +When a pool is created, it is possible to specify two parameters: +- refill size: the minimum size of the memory area to allocate when needed +- align_order: the default alignment to use when reserving memory + +To facilitate the conversion of existing code to pmalloc pools, several +helper functions are provided, mirroring their k/vmalloc counterparts. +However one is missing. There is no pfree() because the memory protected +by a pool will be released exclusively when the pool is destroyed. + + + +Caveats +------- + +- When a pool is protected, whatever memory would be still available in + the current vmap_area (from which allocations are performed) is + relinquished. + +- As already explained, freeing of memory is not supported. Pages will be + returned to the system upon destruction of the memory pool that they + belong to. + +- 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 tobe dynamically allocated and turned into + read-only. + +- 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. + Loading of kernel modules is an exception to this, but it's not expected + to happen with such high frequency to become a problem. + + +Use +--- + +The typical sequence, when using pmalloc, is: + +#. create a pool + + :c:func:`pmalloc_create_pool` + +#. issue one or more allocation requests to the pool + + :c:func:`pmalloc` + + or + + :c:func:`pzalloc` + +#. initialize the memory obtained, with the desired values + +#. write-protect the memory so far allocated + + :c::func:`pmalloc_protect_pool` + +#. iterate over the last 3 points as needed + +#. [optional] destroy the pool + + :c:func:`pmalloc_destroy_pool` + +API +--- + +.. kernel-doc:: include/linux/pmalloc.h +.. kernel-doc:: mm/pmalloc.c -- 2.14.1
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