[PATCH 5/6] Documentation for Pmalloc

[corbet@lwn.net (Jonathan Corbet)]

On Tue, 30 Jan 2018 17:14:45 +0200
Igor Stoppa <igor.stoppa@huawei.com> 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

Please don't put plain-text files into core-api - that's a directory full
of RST documents.  Your document is 99.9% RST already, better to just
finish the job and tie it into the rest of the kernel docs.

> 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 @@

We might as well put the SPDX tag here, it's a new file.

> +============================
> +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.

This is all good information, but I'd suggest it belongs more in the 0/n
patch posting than here.  The introduction of *this* document should say
what it actually covers.

> +
> +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.

This seems like a relevant and important aspect of the API that shouldn't
be buried in the middle of a section talking about random things.

> +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

So one gets this far, but has no actual idea of how to do these things.
Which leads me to wonder: what is this document for?  Who are you expecting
to read it?

You could improve things a lot by (once again) going to RST and using
directives to bring in the kerneldoc comments from the source (which, I
note, do exist).  But I'd suggest rethinking this document and its
audience.  Most of the people reading it are likely wanting to learn how to
*use* this API; I think it would be best to not leave them frustrated.

Thanks,

jon
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