Date: Tue, 30 Jan 2018 17:14:45 +0200 From: Igor Stoppa <igor.stoppa@...wei.com> To: <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>, Igor Stoppa <igor.stoppa@...wei.com> Subject: [PATCH 5/6] Documentation for Pmalloc Detailed documentation about the protectable memory allocator. Signed-off-by: Igor Stoppa <igor.stoppa@...wei.com> --- 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
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