Re: [PATCH v10 1/2] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

On Wed, Jul 3, 2019 at 12:59 AM Andrew Morton [off-list ref] wrote:

On Fri, 28 Jun 2019 11:31:30 +0200 Alexander Potapenko [off-list ref] wrote:

quoted

The new options are needed to prevent possible information leaks and
make control-flow bugs that depend on uninitialized values more
deterministic.

This is expected to be on-by-default on Android and Chrome OS. And it
gives the opportunity for anyone else to use it under distros too via
the boot args. (The init_on_free feature is regularly requested by
folks where memory forensics is included in their threat models.)

init_on_alloc=1 makes the kernel initialize newly allocated pages and heap
objects with zeroes. Initialization is done at allocation time at the
places where checks for __GFP_ZERO are performed.

init_on_free=1 makes the kernel initialize freed pages and heap objects
with zeroes upon their deletion. This helps to ensure sensitive data
doesn't leak via use-after-free accesses.

Both init_on_alloc=1 and init_on_free=1 guarantee that the allocator
returns zeroed memory. The two exceptions are slab caches with
constructors and SLAB_TYPESAFE_BY_RCU flag. Those are never
zero-initialized to preserve their semantics.

Both init_on_alloc and init_on_free default to zero, but those defaults
can be overridden with CONFIG_INIT_ON_ALLOC_DEFAULT_ON and
CONFIG_INIT_ON_FREE_DEFAULT_ON.

If either SLUB poisoning or page poisoning is enabled, those options
take precedence over init_on_alloc and init_on_free: initialization is
only applied to unpoisoned allocations.

Slowdown for the new features compared to init_on_free=0,
init_on_alloc=0:

hackbench, init_on_free=1:  +7.62% sys time (st.err 0.74%)
hackbench, init_on_alloc=1: +7.75% sys time (st.err 2.14%)

Linux build with -j12, init_on_free=1:  +8.38% wall time (st.err 0.39%)
Linux build with -j12, init_on_free=1:  +24.42% sys time (st.err 0.52%)
Linux build with -j12, init_on_alloc=1: -0.13% wall time (st.err 0.42%)
Linux build with -j12, init_on_alloc=1: +0.57% sys time (st.err 0.40%)

The slowdown for init_on_free=0, init_on_alloc=0 compared to the
baseline is within the standard error.

The new features are also going to pave the way for hardware memory
tagging (e.g. arm64's MTE), which will require both on_alloc and on_free
hooks to set the tags for heap objects. With MTE, tagging will have the
same cost as memory initialization.

Although init_on_free is rather costly, there are paranoid use-cases where
in-memory data lifetime is desired to be minimized. There are various
arguments for/against the realism of the associated threat models, but
given that we'll need the infrastructure for MTE anyway, and there are
people who want wipe-on-free behavior no matter what the performance cost,
it seems reasonable to include it in this series.

...

 v10:
  - added Acked-by: tags
  - converted pr_warn() to pr_info()

There are unchangelogged alterations between v9 and v10.  The
replacement of IS_ENABLED(CONFIG_PAGE_POISONING)) with
page_poisoning_enabled().

In the case I send another version of the patch, do I need to
retroactively add them to the changelog?

-- 
Alexander Potapenko
Software Engineer

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