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Subject[PATCH 10/17] prmem: documentation
Documentation for protected memory.

Topics covered:
* static memory allocation
* dynamic memory allocation
* write-rare

Signed-off-by: Igor Stoppa <>
CC: Jonathan Corbet <>
CC: Randy Dunlap <>
CC: Mike Rapoport <>
Documentation/core-api/index.rst | 1 +
Documentation/core-api/prmem.rst | 172 +++++++++++++++++++++++++++++++
3 files changed, 174 insertions(+)
create mode 100644 Documentation/core-api/prmem.rst

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 26b735cefb93..1a90fa878d8d 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -31,6 +31,7 @@ Core utilities
+ prmem

Interfaces for kernel debugging
diff --git a/Documentation/core-api/prmem.rst b/Documentation/core-api/prmem.rst
new file mode 100644
index 000000000000..16d7edfe327a
--- /dev/null
+++ b/Documentation/core-api/prmem.rst
@@ -0,0 +1,172 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. _prmem:
+Memory Protection
+:Date: October 2018
+:Author: Igor Stoppa <>
+- In a typical system using some sort of RAM as execution environment,
+ **all** memory is initially writable.
+- It must be initialized with the appropriate content, be it code or data.
+- Said content typically undergoes modifications, i.e. relocations or
+ relocation-induced changes.
+- The present document doesn't address such transient.
+- Kernel code is protected at system level and, unlike data, it doesn't
+ require special attention.
+Protection mechanism
+- When available, the MMU can write protect memory pages that would be
+ otherwise writable.
+- The protection has page-level granularity.
+- An attempt to overwrite a protected page will trigger an exception.
+- **Write protected data must go exclusively to write protected pages**
+- **Writable data must go exclusively to writable pages**
+Available protections for kernel data
+- **constant**
+ Labelled as **const**, the data is never supposed to be altered.
+ It is statically allocated - if it has any memory footprint at all.
+ The compiler can even optimize it away, where possible, by replacing
+ references to a **const** with its actual value.
+- **read only after init**
+ By tagging an otherwise ordinary statically allocated variable with
+ **__ro_after_init**, it is placed in a special segment that will
+ become write protected, at the end of the kernel init phase.
+ The compiler has no notion of this restriction and it will treat any
+ write operation on such variable as legal. However, assignments that
+ are attempted after the write protection is in place, will cause
+ exceptions.
+- **write rare after init**
+ This can be seen as variant of read only after init, which uses the
+ tag **__wr_after_init**. It is also limited to statically allocated
+ memory. It is still possible to alter this type of variables, after
+ the kernel init phase is complete, however it can be done exclusively
+ with special functions, instead of the assignment operator. Using the
+ assignment operator after conclusion of the init phase will still
+ trigger an exception. It is not possible to transition a certain
+ variable from __wr_ater_init to a permanent read-only status, at
+ runtime.
+- **dynamically allocated write-rare / read-only**
+ After defining a pool, memory can be obtained through it, primarily
+ through the **pmalloc()** allocator. The exact writability state of the
+ memory obtained from **pmalloc()** and friends can be configured when
+ creating the pool. At any point it is possible to transition to a less
+ permissive write status the memory currently associated to the pool.
+ Once memory has become read-only, it the only valid operation, beside
+ reading, is to released it, by destroying the pool it belongs to.
+Protecting dynamically allocated memory
+When dealing with dynamically allocated memory, three options are
+ available for configuring its writability state:
+- **Options selected when creating a pool**
+ When creating the pool, it is possible to choose one of the following:
+ - Writability at allocation time: *WRITABLE*
+ - Writability at protection time: *NONE*
+ - Writability at allocation time: *WRITABLE*
+ - Writability at protection time: *WRITE-RARE*
+ - Writability at allocation time:
+ - the latest allocation: *WRITABLE*
+ - every other allocation: *NONE*
+ - Writability at protection time: *NONE*
+ - Writability at allocation time:
+ - the latest allocation: *WRITABLE*
+ - every other allocation: *WRITE-RARE*
+ - Writability at protection time: *WRITE-RARE*
+ - Writability at allocation time: *WRITE-RARE*
+ - Writability at protection time: *WRITE-RARE*
+ **Remarks:**
+ - The "AUTO" modes perform automatic protection of the content, whenever
+ the current vmap_area is used up and a new one is allocated.
+ - At that point, the vmap_area being phased out is protected.
+ - The size of the vmap_area depends on various parameters.
+ - It might not be possible to know for sure *when* certain data will
+ be protected.
+ - The functionality is provided as tradeoff between hardening and speed.
+ - Its usefulness depends on the specific use case at hand
+ - The "START_WR" mode is the only one which provides immediate protection, at the cost of speed.
+- **Protecting the pool**
+ This is achieved with **pmalloc_protect_pool()**
+ - Any vmap_area currently in the pool is write-protected according to its initial configuration.
+ - Any residual space still available from the current vmap_area is lost, as the area is protected.
+ - **protecting a pool after every allocation will likely be very wasteful**
+ - Using PMALLOC_MODE_START_WR is likely a better choice.
+- **Upgrading the protection level**
+ This is achieved with **pmalloc_make_pool_ro()**
+ - it turns the present content of a write-rare pool into read-only
+ - can be useful when the content of the memory has settled
+- Freeing of memory is not supported. Pages will be returned to the
+ system upon destruction of their memory pool.
+- The address range available for vmalloc (and thus for pmalloc too) is
+ limited, on 32-bit systems. However it shouldn't be an issue, since not
+ much data is expected to be dynamically allocated and turned into
+ write-protected.
+- Regarding SMP systems, changing state of pages and altering mappings
+ requires performing cross-processor synchronizations of page tables.
+ This is an additional reason for limiting the use of write rare.
+- Not only the pmalloc memory must be protected, but also any reference to
+ it that might become the target for an attack. The attack would replace
+ a reference to the protected memory with a reference to some other,
+ unprotected, memory.
+- The users of rare write must take care of ensuring the atomicity of the
+ action, respect to the way they use the data being altered; for example,
+ take a lock before making a copy of the value to modify (if it's
+ relevant), then alter it, issue the call to rare write and finally
+ release the lock. Some special scenario might be exempt from the need
+ for locking, but in general rare-write must be treated as an operation
+ that can incur into races.
+- pmalloc relies on virtual memory areas and will therefore use more
+ tlb entries. It still does a better job of it, compared to invoking
+ vmalloc for each allocation, but it is undeniably less optimized wrt to
+ TLB use than using the physmap directly, through kmalloc or similar.
+**add examples here**
+.. kernel-doc:: include/linux/prmem.h
+.. kernel-doc:: mm/prmem.c
+.. kernel-doc:: include/linux/prmemextra.h
index ea979a5a9ec9..246b1a1cc8bb 100644
@@ -9463,6 +9463,7 @@ F: include/linux/prmemextra.h
F: mm/prmem.c
F: mm/test_write_rare.c
F: mm/test_pmalloc.c
+F: Documentation/core-api/prmem.rst

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