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    SubjectRe: [PATCH 5/6] Documentation for Pmalloc
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    Date
    2 Minor typos inline below:

    On 01/24/2018 09:56 AM, Igor Stoppa wrote:
    > Detailed documentation about the protectable memory allocator.
    >
    > Signed-off-by: Igor Stoppa <igor.stoppa@huawei.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..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

    obtained

    > +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

    through

    > +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.
    >

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