I attended this year's Linux Kernel Summit in Santa Fe, NM, USA and
made notes on some of the sessions that were relevant to Debian.
LWN also
reported many of the discussions.
This is the second and last part of my notes;
part 1 is here.
Kernel Hardening
Kees Cook presented the ongoing work on upstream kernel hardening,
also known as the
Kernel
Self-Protection Project or KSPP.
GCC plugins
The kernel build system can now build and use GCC plugins to
implement some protections. This requires gcc 4.5 and the plugin
headers installed. It has been tested on x86, arm, and arm64. It
is disabled by CONFIG_COMPILE_TEST because CI
systems using allmodconfig/allyesconfig probably don't have those
installed, but this ought to be changed at some point.
There was a question as to how plugin headers should be installed
for cross-compilers or custom compilers, but I didn't hear a clear
answer to this. Kees has been prodding distribution gcc maintainers
to package them. Mark Brown mentioned the Linaro toolchain being
widely used; Kees has not talked to its maintainers yet.
Probabilistic protections
These protections are based on hidden state that an attacker will
need to discover in order to make an effective attack; they
reduce the probability of success but don't prevent it
entirely.
Kernel address space layout randomisation (KASLR) has now been
implemented on x86, arm64, and mips for the kernel image.
(Debian enables this.) However there are still lots of information
leaks that defeat this. This could theoretically be improved by
relocating different sections or smaller parts of the kernel
independently, but this requires re-linking at boot. Aside from
software information leaks, the branch target predictor on (common
implementations of) x86 provides a side channel to find addresses of
branches in the kernel.
Page and heap allocation, etc., is still quite predictable.
struct randomisation (RANDSTRUCT plugin from grsecurity)
reorders members in (a) structures containing only function pointers
(b) explicitly marked structures. This makes it very hard to attack
custom kernels where the kernel image is not readable. But even for
distribution kernels, it increases the maintenance burden for
attackers.
Deterministic protections
These protections block a class of attacks completely.
Read-only protection of kernel memory is either mandatory or enabled
by default on x86, arm, and arm64. (Debian enables this.)
Protections against execution of user memory in kernel mode are now
implemented in hardware on x86 (SMEP, in Intel processors from
Skylake onward) and on arm64 (PXN, from ARMv8.1). But Skylake is
not available for servers and ARMv8.1 is not yet implemented at all!
s390 always had this protection.
It may be possible to 'emulate' this using other hardware
protections. arm (v7) and arm64 now have this, but x86 doesn't.
Linus doesn't like the overhead of previously proposed
implementations for x86. It is possible to do this using PCID (in
Intel processors from Sandy Bridge onward), which has already been
done in PaX - and this should be fast enough.
Virtually mapped stacks protect against stack overflow attacks.
They were implemented as an option for x86 only in 4.9.
(Debian enables this.)
Copies to or from user memory sometimes use a user-controlled size
that is not properly bounded. Hardened usercopy, implemented as an
option in 4.8 for many architectures, protects against this.
(Debian enables this.)
Memory wiping (zero on free) protects against some information leaks
and use-after-free bugs. It was already implemented as debug feature with
non-zero poison value, but at some performance cost. Zeroing can
be cheaper since it allows allocator to skip zeroing on
reallocation. That was implemented as an option in 4.6.
(Debian does not currently enable this but we might do if the
performance cost is low enough.)
Constification (with the CONSTIFY gcc plugin) reduces the
amount of static data that can be written to. As
with RANDSTRUCT, this is applied to function pointer tables
and explicitly marked structures. Instances of some types need to
be modified very occasionally. In PaX/Grsecurity this is done with
pax_{open,close}_kernel() which
globally disable write protection temporarily. It would be preferable
to override write protection in a more directed way, so that the
permission to write doesn't leak into any other code that interrupts
this process. The feature is not in mainline yet.
Atomic wrap detction protects against reference-counting bugs which
can result in a use-after-free. Overflow and underflow are trapped
and result in an 'oops'. There is no measurable performance
impact. It would be applied to all operations on
the atomic_t type, but there needs to be an opt-out for
atomics that are not ref-counters - probably by adding
an atomic_wrap_t type for them. This has been implemented
for x86, arm, and arm64 but is not in mainline yet.
Kernel Freezer Hell
For the second year running, Jiri Kosina raised the problem of
'freezing' kthreads (kernel-mode threads) in preparation for system
suspend (suspend to RAM, or hibernation). What are the semantics?
What invariants should be met when a kthread gets frozen? They are
not defined anywhere.
Most freezable threads don't actually need to be quiesced. Also
many non-freezable threads are pointlessly
calling try_to_freeze() (probably due to copying code
without understanding it)).
At a system level, what we actually need is I/O and filesystem
consistency. This should be achieved by:
Telling mounted filesystems to freeze. They can quiesce any
kthreads they created.
Device drivers quiescing any kthreads they created, from their
PM suspend implementation.
The system suspend code should not need to directly freeze threads.
Kernel Documentation
Jon Corbet and Mauro Carvalho presented the recent work on
kernel documentation.
The kernel's documentation system was a house of cards
involving DocBook and a lot of custom scripting. Both the DocBook
templates and plain text files are gradually being converted
to reStructuredText
format, processed
by Sphinx. However, manual
page generation is currently 'broken' for documents processed by
Sphinx.
There are about 150 files at the top level of the documentation
tree, that are being gradually moved into subdirectories. The most
popular files, that are likely to be referenced in external
documentation, have been replaced by placeholders.
Sphinx is highly extensible and this has been used to integrate
kernel-doc.
It would be possible to add extensions that parse and include the
MAINTAINERS file and Documentation/ABI/ files,
which have their own formats, but the documentation maintainers
would prefer not to add extensions that can't be pushed to Sphinx
upstream.
There is lots of obsolete documentation, and patches to remove those
would be welcome.
Linus objected to PDF files recently added under
the Documentation/media directory - they are not the source
format so should not be there! They should be generated from the
corresponding SVG or image files at build time.
Issues around Tracepoints
Steve Rostedt and Shuah Khan led a discussion about tracepoints.
Currently each maintainer decides which tracepoints to create. The
cost of each added tracepoint is minimal, but the cost of very many
tracepoints is more substantial. So there is such a thing as too
many tracepoints, and we need a policy to decide when they are
justified. They advised not to create tracepoints just in
case, since kprobes can be used for tracing (almost) anywhere
dynamically.
There was some support for requiring documentation of each new
tracepoint. That may dissuade introduction of obscure tracepoints,
but also creates a higher expectation of stability.
Tools such as bcc and IOVisor are now being created that depend on
specific tracepoints or even function names (through kprobes).
Should we care about breaking them?
Linus said that we should strive to be polite to developers
and users relying on tracepoints, but if it's too painful to
maintain a tracepoint then we should go ahead and change it. Where
the end users of the tool are themselves developers it's more
reasonable to expect them to upgrade the tool and we should care
less about changing it. In some cases tracepoints could provide
dummy data for compatibility (as is done in some places in procfs).