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Date: Sun, 4 Feb 2018 13:37:24 -0800
From: Randy Dunlap <rdunlap@...radead.org>
To: Igor Stoppa <igor.stoppa@...wei.com>, jglisse@...hat.com,
keescook@...omium.org, mhocko@...nel.org, labbott@...hat.com,
hch@...radead.org, willy@...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
Subject: Re: [PATCH 6/6] Documentation for Pmalloc
Hi,
On 02/04/2018 09:00 AM, Igor Stoppa wrote:
> 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 d5bbe035316d..7244ddeb540f 100644
> --- a/Documentation/core-api/index.rst
> +++ b/Documentation/core-api/index.rst
> @@ -21,6 +21,7 @@ Core utilities
> flexible-arrays
> librs
> genalloc
> + 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..8dabb5e18d8f
> --- /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 it could take advantage
nor could it
> +of the qualifier __ro_after_init, but is write-once and read-only in spirit.
> +It protects data from both accidental and malicious overwrites.
> +
> +Ex: A policy that is loaded from userspace.
Either
Example:
or
E.g.:
(meaning For example)
> +
> +
> +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 idrivers and threads can use different pools, for finer
drivers
> +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 requestor, however it will not become avaiable for
requester; however, available
> + 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 wrt allocating vs protecting
Write out "wrt" -> with respect to.
> + an individual pool, for performance reason. It is recommended to not
reasons. 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 directly vmalloc. The exact number depends on size
of using vmalloc directly. 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
write-protect
> +
> +.. kernel-doc:: include/linux/pmalloc.h
> + :functions: pmalloc_protect_pool
> +
> +7. use in read-only mode the handlers obtained through the allocations
handles ??
> +
> +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
>
--
~Randy
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