Date: Sun, 4 Feb 2018 13:37:24 -0800 From: Randy Dunlap <rdunlap@...radead.org> To: Igor Stoppa <igor.stoppa@...wei.com>, 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 Subject: Re: [PATCH 6/6] Documentation for Pmalloc Hi, On 02/04/2018 09:00 AM, Igor Stoppa wrote: > Detailed documentation about the protectable memory allocator. > > Signed-off-by: Igor Stoppa <igor.stoppa@...wei.com> > --- > Documentation/core-api/index.rst | 1 + > Documentation/core-api/pmalloc.rst | 114 +++++++++++++++++++++++++++++++++++++ > 2 files changed, 115 insertions(+) > create mode 100644 Documentation/core-api/pmalloc.rst > > diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst > index d5bbe035316d..7244ddeb540f 100644 > --- a/Documentation/core-api/index.rst > +++ b/Documentation/core-api/index.rst > @@ -21,6 +21,7 @@ Core utilities > flexible-arrays > librs > genalloc > + pmalloc > > Interfaces for kernel debugging > =============================== > diff --git a/Documentation/core-api/pmalloc.rst b/Documentation/core-api/pmalloc.rst > new file mode 100644 > index 000000000000..8dabb5e18d8f > --- /dev/null > +++ b/Documentation/core-api/pmalloc.rst > @@ -0,0 +1,114 @@ > +SPDX-License-Identifier: GPL-2.0 > + > +Protectable memory allocator > +============================ > + > +Purpose > +------- > + > +The pmalloc library is meant to provide R/O status to data that, for some > +reason, could neither be declared as constant, nor it could take advantage nor could it > +of the qualifier __ro_after_init, but is write-once and read-only in spirit. > +It protects data from both accidental and malicious overwrites. > + > +Ex: A policy that is loaded from userspace. Either Example: or E.g.: (meaning For example) > + > + > +Concept > +------- > + > +pmalloc builds on top of genalloc, using the same concept of memory pools. > + > +The value added by pmalloc is that now the memory contained in a pool can > +become R/O, for the rest of the life of the pool. > + > +Different kernel idrivers and threads can use different pools, for finer drivers > +control of what becomes R/O and when. And for improved lockless concurrency. > + > + > +Caveats > +------- > + > +- Memory freed while a pool is not yet protected will be reused. > + > +- Once a pool is protected, it's not possible to allocate any more memory > + from it. > + > +- Memory "freed" from a protected pool indicates that such memory is not > + in use anymore by the requestor, however it will not become avaiable for requester; however, available > + further use, until the pool is destroyed. > + > +- Before destroying a pool, all the memory allocated from it must be > + released. > + > +- pmalloc does not provide locking support wrt allocating vs protecting Write out "wrt" -> with respect to. > + an individual pool, for performance reason. It is recommended to not reasons. not to > + share the same pool between unrelated functions. Should sharing be a > + necessity, the user of the shared pool is expected to implement locking > + for that pool. > + > +- pmalloc uses genalloc to optimize the use of the space it allocates > + through vmalloc. Some more TLB entries will be used, however less than > + in the case of using directly vmalloc. The exact number depends on size of using vmalloc directly. on the size > + of each allocation request and possible slack. > + > +- 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. > + > +- To facilitate the conversion of existing code to pmalloc pools, several > + helper functions are provided, mirroring their kmalloc counterparts. > + > + > +Use > +--- > + > +The typical sequence, when using pmalloc, is: > + > +1. create a pool > + > +.. kernel-doc:: include/linux/pmalloc.h > + :functions: pmalloc_create_pool > + > +2. [optional] pre-allocate some memory in the pool > + > +.. kernel-doc:: include/linux/pmalloc.h > + :functions: pmalloc_prealloc > + > +3. issue one or more allocation requests to the pool with locking as needed > + > +.. kernel-doc:: include/linux/pmalloc.h > + :functions: pmalloc > + > +.. kernel-doc:: include/linux/pmalloc.h > + :functions: pzalloc > + > +4. initialize the memory obtained with desired values > + > +5. [optional] iterate over points 3 & 4 as needed > + > +6. write protect the pool write-protect > + > +.. kernel-doc:: include/linux/pmalloc.h > + :functions: pmalloc_protect_pool > + > +7. use in read-only mode the handlers obtained through the allocations handles ?? > + > +8. [optional] release all the memory allocated > + > +.. kernel-doc:: include/linux/pmalloc.h > + :functions: pfree > + > +9. [optional, but depends on point 8] destroy the pool > + > +.. kernel-doc:: include/linux/pmalloc.h > + :functions: pmalloc_destroy_pool > + > +API > +--- > + > +.. kernel-doc:: include/linux/pmalloc.h > -- ~Randy
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