From patchwork Wed Jan 24 17:56:30 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Igor Stoppa X-Patchwork-Id: 10182933 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 54979601D5 for ; Wed, 24 Jan 2018 17:59:35 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 393F3288B3 for ; Wed, 24 Jan 2018 17:59:35 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 2DD462890C; Wed, 24 Jan 2018 17:59:35 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.2 required=2.0 tests=BAYES_00, RCVD_IN_DNSWL_MED autolearn=ham version=3.3.1 Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.wl.linuxfoundation.org (Postfix) with SMTP id 0F393288EF for ; Wed, 24 Jan 2018 17:59:33 +0000 (UTC) Received: (qmail 1805 invoked by uid 550); 24 Jan 2018 17:59:31 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Delivered-To: mailing list kernel-hardening@lists.openwall.com Received: (qmail 1783 invoked from network); 24 Jan 2018 17:59:31 -0000 From: Igor Stoppa To: , , , , , CC: , , , , , Igor Stoppa Date: Wed, 24 Jan 2018 19:56:30 +0200 Message-ID: <20180124175631.22925-6-igor.stoppa@huawei.com> X-Mailer: git-send-email 2.9.3 In-Reply-To: <20180124175631.22925-1-igor.stoppa@huawei.com> References: <20180124175631.22925-1-igor.stoppa@huawei.com> MIME-Version: 1.0 X-Originating-IP: [10.122.225.51] X-CFilter-Loop: Reflected Subject: [kernel-hardening] [PATCH 5/6] Documentation for Pmalloc X-Virus-Scanned: ClamAV using ClamSMTP Detailed documentation about the protectable memory allocator. Signed-off-by: Igor Stoppa --- Documentation/core-api/pmalloc.txt | 104 +++++++++++++++++++++++++++++++++++++ 1 file changed, 104 insertions(+) create mode 100644 Documentation/core-api/pmalloc.txt diff --git a/Documentation/core-api/pmalloc.txt b/Documentation/core-api/pmalloc.txt new file mode 100644 index 0000000..9c39672 --- /dev/null +++ b/Documentation/core-api/pmalloc.txt @@ -0,0 +1,104 @@ +============================ +Protectable memory allocator +============================ + +Introduction +------------ + +When trying to perform an attack toward a system, the attacker typically +wants to alter the execution flow, in a way that allows actions which +would otherwise be forbidden. + +In recent years there has been lots of effort in preventing the execution +of arbitrary code, so the attacker is progressively pushed to look for +alternatives. + +If code changes are either detected or even prevented, what is left is to +alter kernel data. + +As countermeasure, constant data is collected in a section which is then +marked as readonly. +To expand on this, also statically allocated variables which are tagged +as __ro_after_init will receive a similar treatment. +The difference from constant data is that such variables can be still +altered freely during the kernel init phase. + +However, such solution does not address those variables which could be +treated essentially as read-only, but whose size is not known at compile +time or cannot be fully initialized during the init phase. + + +Design +------ + +pmalloc builds on top of genalloc, using the same concept of memory pools +A pool is a handle to a group of chunks of memory of various sizes. +When created, a pool is empty. It will be populated by allocating chunks +of memory, either when the first memory allocation request is received, or +when a pre-allocation is performed. + +Either way, one or more memory pages will be obtaiend from vmalloc and +registered in the pool as chunk. Subsequent requests will be satisfied by +either using any available free space from the current chunks, or by +allocating more vmalloc pages, should the current free space not suffice. + +This is the key point of pmalloc: it groups data that must be protected +into a set of pages. The protection is performed through the mmu, which +is a prerequisite and has a minimum granularity of one page. + +If the relevant variables were not grouped, there would be a problem of +allowing writes to other variables that might happen to share the same +page, but require further alterations over time. + +A pool is a group of pages that are write protected at the same time. +Ideally, they have some high level correlation (ex: they belong to the +same module), which justifies write protecting them all together. + +To keep it to a minimum, locking is left to the user of the API, in +those cases where it's not strictly needed. +Ideally, no further locking is required, since each module can have own +pool (or pools), which should, for example, avoid the need for cross +module or cross thread synchronization about write protecting a pool. + +The overhead of creating an additional pool is minimal: a handful of bytes +from kmalloc space for the metadata and then what is left unused from the +page(s) registered as chunks. + +Compared to plain use of vmalloc, genalloc has the advantage of tightly +packing the allocations, reducing the number of pages used and therefore +the pressure on the TLB. The slight overhead in execution time of the +allocation should be mostly irrelevant, because pmalloc memory is not +meant to be allocated/freed in tight loops. Rather it ought to be taken +in use, initialized and write protected. Possibly destroyed. + +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 systemd. + +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. + + +Use +--- + +The typical sequence, when using pmalloc, is: + +1. create a pool +2. [optional] pre-allocate some memory in the pool +3. issue one or more allocation requests to the pool +4. initialize the memory obtained + - iterate over points 3 & 4 as needed - +5. write protect the pool +6. use in read-only mode the handlers obtained throguh the allocations +7. [optional] destroy the pool + + +In a scenario where, for example due to some error, part or all of the +allocations performed at point 3 must be reverted, it is possible to free +them, as long as point 5 has not been executed, and the pool is still +modifiable. Such freed memory can be re-used. +Performing a free operation on a write-protected pool will, instead, +simply release the corresponding memory from the accounting, but it will +be still impossible to alter its content.