[PATCH 5/6] Documentation for Pmalloc

[Igor Stoppa]

Detailed documentation about the protectable memory allocator.

Signed-off-by: Igor Stoppa <igor.stoppa@xxxxxxxxxx>
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 Documentation/core-api/pmalloc.txt | 104 +++++++++++++++++++++++++++++++++++++
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 create mode 100644 Documentation/core-api/pmalloc.txt

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+============================
+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