[PATCH v5 00/11] archs/random: fallback to best raw ktime when no cycle counter

Jason A. Donenfeld
2022-04-19T11:17:44Z

Hi folks,

The RNG uses a function called random_get_entropy() basically anytime
that it needs to timestamp an event. For example, an interrupt comes in,
and we mix a random_get_entropy() into the entropy pool somehow.
Somebody mashes their keyboard or moves their mouse around? We mix a
random_get_entropy() into the entropy pool. It's one of the main
varieties of input.

Unfortunately, it's always 0 on a few platforms. The RNG has accumulated
various hacks to deal with this, but in general it's not great. Surely
we can do better than 0. In fact, *anything* that's not the same exact
value all the time would be better than 0. Even a counter that
increments once per hour would be better than 0! I think you get the
idea.

On most platforms, random_get_entropy() is aliased to get_cycles(),
which makes sense for platforms where get_cycles() is defined. RDTSC,
for example, has all the characteristics we care about for this
function: it's fast to acquire (i.e. acceptable in an irq handler),
pretty high precision, available, forms a 2-monotone distribution, etc.
But for platforms without that, what is the next best thing?

Sometimes the next best thing is architecture-defined. For example,
really old MIPS has the C0 random register, which isn't quite a cycle
counter, but is at least something. However, some platforms don't even
have an architecture-defined fallback.

Fortunately, the timekeeping subsystem has already solved this problem
of trying to determine what the least bad clock is on constrained
systems, falling back to jiffies in the worst case. By exporting the raw
clock, we can get a decent fallback function for when there's no cycle
counter or architecture-specific function.

This series makes the RNG more useful on: m68k, RISC-V, MIPS, ARM32,
NIOS II, SPARC32, Xtensa, and Usermode Linux. Previously these platforms
would, in certain circumstances, but out of luck with regards to having
any type of event timestamping source in the RNG.

Finally, note that this series isn't about "jitter entropy" or other
ways of initializing the RNG. That's a different topic for a different
thread. Please don't let this discussion veer off into that. Here, I'm
just trying to find a good fallback counter/timer for platforms without
get_cycles(), a question with limited scope.

If this (or a future revision) looks good to you all and receives the
requisite acks, my plan was to take these through the random.git tree
for 5.19, so that I can then build on top of it.

Thanks,
Jason

Changes v4->v5:
- Do not prototype symbol with 'extern', according to style guide.
- On MIPS, combine random_get_entropy_fallback() with the c0 random
register in a way that matches the format of the c0 random value, so
that we get the best of a high precision cycle counter and of larger
period timer, joined together. As a result, Thomas Bogendoerfer's
ack on v4 of patch 4 has been dropped, since this is a substantial
change.

Changes v3->v4:
- Use EXPORT_SYMBOL_GPL instead of EXPORT_SYMBOL.

Changes v2->v3:
- Name the fallback function random_get_entropy_fallback(), so that it
can be changed out as needed.
- Include header with prototype in timekeeping.c to avoid compiler
warning.
- Export fallback function symbol.

Changes v1->v2:
- Use ktime_read_raw_clock() instead of sched_clock(), per Thomas'
suggestion.
- Drop arm64 change.
- Cleanup header inclusion ordering problem.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: David S. Miller <davem@davemloft.net>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-m68k@lists.linux-m68k.org
Cc: linux-mips@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Cc: sparclinux@vger.kernel.org
Cc: linux-um@lists.infradead.org
Cc: x86@kernel.org
Cc: linux-xtensa@linux-xtensa.org

Jason A. Donenfeld (11):
timekeeping: add raw clock fallback for random_get_entropy()
m68k: use fallback for random_get_entropy() instead of zero
riscv: use fallback for random_get_entropy() instead of zero
mips: use fallback for random_get_entropy() instead of just c0 random
arm: use fallback for random_get_entropy() instead of zero
nios2: use fallback for random_get_entropy() instead of zero
x86: use fallback for random_get_entropy() instead of zero
um: use fallback for random_get_entropy() instead of zero
sparc: use fallback for random_get_entropy() instead of zero
xtensa: use fallback for random_get_entropy() instead of zero
random: insist on random_get_entropy() existing in order to simplify

arch/arm/include/asm/timex.h | 1 +
arch/m68k/include/asm/timex.h | 2 +-
arch/mips/include/asm/timex.h | 16 +++---
arch/nios2/include/asm/timex.h | 2 +
arch/riscv/include/asm/timex.h | 2 +-
arch/sparc/include/asm/timex_32.h | 4 +-
arch/um/include/asm/timex.h | 9 +---
arch/x86/include/asm/timex.h | 10 ++++
arch/xtensa/include/asm/timex.h | 6 +--
drivers/char/random.c | 89 ++++++++++---------------------
include/linux/timex.h | 8 +++
kernel/time/timekeeping.c | 10 ++++
12 files changed, 74 insertions(+), 85 deletions(-)

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2.35.1