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Message-ID: <98b2fecf-c1b3-aa5e-ba70-2770940bb965@gmail.com>
Date: Fri, 23 Feb 2018 16:26:49 -0800
From: J Freyensee <why2jjj.linux@...il.com>
To: Igor Stoppa <igor.stoppa@...wei.com>, 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
Subject: Re: [PATCH 7/7] Documentation for Pmalloc
On 2/23/18 6:48 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 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.
Is that true? pmalloc_destroy_pool() has:
.
.
+ pmalloc_pool_set_protection(pool, false);
+ gen_pool_for_each_chunk(pool, pmalloc_chunk_free, NULL);
+ gen_pool_destroy(pool);
+ kfree(data);
which to me looks like is the opposite, the data (ie, "memory") is being
released first, then the pool is destroyed.
> +
> +- pmalloc does not provide locking support with respect to allocating vs
> + protecting an individual pool, for performance reasons.
What is the recommendation to using locks then, as the computing
real-world mainly operates in multi-threaded/process world? Maybe show
an example of an issue that occur if locks aren't used and give a coding
example.
> + 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.
Why is 32-bit systems mentioned and not 64-bit? Is there a problem with
64-bit here?
Thanks,
Jay
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