Re: [PATCH 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

From: Ingo Molnar
Date: Tue Feb 06 2018 - 03:49:01 EST


* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> [...] so I implemented a real, per function register usage tracking.
>
> For the x86 defconfig kernel the results are:
>
> r11: used in 1704 fns, not used in 43310 fns, usage ratio: 3.8%
> r10: used in 3809 fns, not used in 41205 fns, usage ratio: 8.5%
> r15: used in 6599 fns, not used in 38415 fns, usage ratio: 14.7%
> r9: used in 8120 fns, not used in 36894 fns, usage ratio: 18.0%
> r14: used in 9243 fns, not used in 35771 fns, usage ratio: 20.5%
> r8: used in 12614 fns, not used in 32400 fns, usage ratio: 28.0%
> r13: used in 12708 fns, not used in 32306 fns, usage ratio: 28.2%
> r12: used in 17144 fns, not used in 27870 fns, usage ratio: 38.1%
> rbp: used in 23289 fns, not used in 21725 fns, usage ratio: 51.7%
> rcx: used in 23897 fns, not used in 21117 fns, usage ratio: 53.1%
> rbx: used in 29226 fns, not used in 15788 fns, usage ratio: 64.9%
> rdx: used in 33205 fns, not used in 11809 fns, usage ratio: 73.8%
> rsi: used in 35415 fns, not used in 9599 fns, usage ratio: 78.7%
> rdi: used in 40628 fns, not used in 4386 fns, usage ratio: 90.3%
> rax: used in 43120 fns, not used in 1894 fns, usage ratio: 95.8%

So here's the next (and probably final) chapter of x86-64 register allocation
statistics: out of curiosity I let this analysis run overnight on all 4 kernel
configs, to see the register usage patterns of the distro and allyesconfig kernels
as well.

Here's all the per function register allocation probabilities in a single table:

REG allnoconfig localconfig distroconfig allyesconfig
--------------------------------------------------------------------------
rax: 94.6% 95.8% 94.3% 96.2%
rbx: 46.9% 64.9% 67.6% 90.4%
rcx: 47.8% 53.1% 57.9% 52.7%
rdx: 66.0% 73.8% 76.0% 74.3%
rbp: 36.2% 51.7% 55.5% 81.5%
rsi: 64.8% 78.7% 81.3% 85.0%
rdi: 79.9% 90.3% 92.1% 94.3%
r8: 21.9% 28.0% 31.9% 29.7%
r9: 13.9% 18.0% 20.4% 18.3%
r10: 9.3% 8.5% 8.4% 4.7%
r11: 4.9% 3.8% 4.5% 1.6%
r12: 25.6% 38.1% 42.4% 69.3%
r13: 18.3% 28.2% 31.5% 57.1%
r14: 13.3% 20.5% 22.8% 46.1%
r15: 9.3% 14.7% 16.4% 36.6%

These numbers underline the overall conclusions that we have reached so far:

- We should clear all of R10-R15 in syscalls and R8-R15 in parameter-less
entries (IRQs, NMIs, exceptions, etc.) - like the latest series from Dan does.

- We should probably strive to clear R8-R9 for system calls that don't use it -
which is ~98% of them. In particular R9 with its comparatively low (~20%)
allocation probability could survive deep into the kernel: 5-deep call chains
still have a ~30% chance to have R9 intact - and call chains as deep as 10
could still realistically have a ~10% residual probability to have R9 intact.
We don't do this yet.

- Smaller kernels are statistically easier to attack via Spectre, as long as the
gadget is present in the smaller kernel. In particular heavily stripped down
64-bit kernels might be attackable via R8-R9 (21%,14%) and also RBP (36%) to a
certain degree. This means that the RBP clearing introduced by this series is
very much relevant: because RBP is not part of the C function call calling
arguments ABI its allocation frequency is much lower than that of other GP
registers. Unfortunately R8/R9 values will survive through system calls,
because we restore them in do_syscall_64().

There's a somewhat surprising pattern as well: the register allocation probability
of R10 and R11 _decreases_ as the kernel gets more complex. For all other
registers the allocation probability increases with increasing kernel complexity,
which is intuitive: larger functions with higher register pressure will use more
registers.

So this result is counter-intuitive - my best guess is that it's some sort of GCC
register allocation artifact. Here's the comparison of the code generation of a
distro versus an allyesconfig kernel:

# distro-config allyesconfig
#
# nr of =y .config options: 4871 9553
# nr of functions: 190477 249340
# nr of instructions: 10329411 20223765
# nr of register uses: 16907185 33413619
#
# instructions per function: 54 81
#
#
# r10 used in: 15404 fns 11300 fns
# r10 not used in: 167714 fns 228114 fns
# r10 usage ratio: 8.4% 4.7%
#
# r11 used in: 8224 fns 3876 fns
# r11 not used in: 174894 fns 235538 fns
# r11 usage ratio: 4.5% 1.6%

I don't know which kernel option (out of thousands) causes R10/R11 to be used much
less frequently in a significantly larger kernel.

Note that even the absolute count of functions with R10/R11 use decreases in the
allyesconfig kernel, so I don't think it can be caused by the extra
instrumentation bloat of features like CONFIG_GCOV_KERNEL=y.

The basic inlining and optimization settings are the same and neither has
branch-instrumentation enabled:

# distro-config allyesconfig
#
CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING=y CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y
# CONFIG_CC_OPTIMIZE_FOR_SIZE is not set # CONFIG_CC_OPTIMIZE_FOR_SIZE is not set
CONFIG_OPTIMIZE_INLINING=y CONFIG_OPTIMIZE_INLINING=y

CONFIG_BRANCH_PROFILE_NONE=y CONFIG_BRANCH_PROFILE_NONE=y

While no-one will build and boot an allyesconfig kernel (other than me), the
numbers are still indicative: we should keep in mind the possibility that a Linux
distro enabling seemingly benign non-default kernel options can lower the
allocation probability of R10/R11 significantly.

Thanks,

Ingo