From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AH8x226ABm5dybQtEW3vH760nfUr3UZFwPVPIeNeZDrCmvgm23jUtwB7EbYHDu6A0SiYULk4AGqC ARC-Seal: i=1; a=rsa-sha256; t=1517325492; cv=none; d=google.com; s=arc-20160816; b=raNby7+zQVSG+ObKeMRhHAQqlduTPVz0ihMKWoSKe4P0zEO5ADTa48u/eLxx8IKybu ccSV5kiuXCB8RP6zyNKY1Sx92SvH7hM9Z9kaOXjYVLNrZ8+2Hc/zopnpP9UN3IdjHMLz 24cFtHfEfrvqYEqdqK8s5ulg9xlqVqVeb1UAvJu2KWj4mnMSCXJTWCiedu9vViiOPqN5 4wBjm1AebyGMSttxUls3zfKG7mMEkVj85CfqLdbb9VjP4iYNJ6AzeqxEUDMS/aonHh09 yRCxFBnYuETJk+1D/z1wXYQ4zRg51lM9HxpJ/aaXtqOGa8TjBfpQgN5Uu9km5qBvhKzk K42Q== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=mime-version:references:in-reply-to:message-id:date:subject:cc:to :from:delivered-to:list-id:list-subscribe:list-unsubscribe:list-help :list-post:precedence:mailing-list:arc-authentication-results; bh=NiL2gW0ZDXsC4JCVPRqSeDaDXgNQIwaCemRsMv0L9RM=; b=gPxbWvveW9ga4gozhV23YK7FB317sQJEIfEBNonWvlDS8a8GKoNGTNn1i5UszvjzW8 anZM5FpmiWjBFrj4s40PkNPrMmJ1h4feKtFwnZEStSEsoHFvJ4+ry9xY9DDoDbzfmA2t Rspe+KOWMZxo9tr/FRoIODddxb1tMivcy9xqmGkO/DgqtagtCiswAnX3SmYlJOOI1zq8 y43wD0SCn3AzFbeiAfy1NlKOirKgfsTEnqBBaPlRyEPV4kRWXQ2Q09jTBbmsxZWano+7 2W1ctWuC97WgDz3BsdR8g+S3vCcWuRGVTsZhgI8288SO+zjmKgCj7qsE8908AivQvE3d AMrQ== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of kernel-hardening-return-11534-gregkh=linuxfoundation.org@lists.openwall.com designates 195.42.179.200 as permitted sender) smtp.mailfrom=kernel-hardening-return-11534-gregkh=linuxfoundation.org@lists.openwall.com Authentication-Results: mx.google.com; spf=pass (google.com: domain of kernel-hardening-return-11534-gregkh=linuxfoundation.org@lists.openwall.com designates 195.42.179.200 as permitted sender) smtp.mailfrom=kernel-hardening-return-11534-gregkh=linuxfoundation.org@lists.openwall.com Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm List-Post: List-Help: List-Unsubscribe: List-Subscribe: From: Igor Stoppa To: , , , , , CC: , , , , , Igor Stoppa Subject: [PATCH 5/6] Documentation for Pmalloc Date: Tue, 30 Jan 2018 17:14:45 +0200 Message-ID: <20180130151446.24698-6-igor.stoppa@huawei.com> X-Mailer: git-send-email 2.9.3 In-Reply-To: <20180130151446.24698-1-igor.stoppa@huawei.com> References: <20180130151446.24698-1-igor.stoppa@huawei.com> MIME-Version: 1.0 Content-Type: text/plain X-Originating-IP: [10.122.225.51] X-CFilter-Loop: Reflected X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1591031095823992296?= X-GMAIL-MSGID: =?utf-8?q?1591031095823992296?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: 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..934d356 --- /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 obtained 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 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. + + +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 through 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. -- 2.9.3