Fuchsia

Zircon Kernel Commandline Options

TODO(fxbug.dev/53594): move kernel_cmdline.md verbiage here

Options common to all machines

kernel.entropy-mixin=<hexadecimal>

Provides entropy to be mixed into the kernel's CPRNG. The value must be a string of lowercase hexadecimal digits.

The original value will be scrubbed from memory as soon as possible and will be redacted from all diagnostic output.

kernel.serial=[none | legacy | qemu | <type>,<base>,<irq>]

Default: none

TODO(53594)

Options available only on x86 machines

kernel.x86.disable_spec_mitigations=<bool>

Default: false

If set, disables all speculative execution information leak mitigations.

If unset, the per-mitigation defaults will be used.

kernel.x86.md_clear_on_user_return=<bool>

Default: true

MDS (Microarchitectural Data Sampling) is a family of speculative execution information leak bugs that allow the contents of recent loads or stores to be inferred by hostile code, regardless of privilege level (CVE-2019-11091, CVE-2018-12126, CVE-2018-12130, CVE-2018-12127). For example, this could allow user code to read recent kernel loads/stores.

To avoid this bug, it is required that all microarchitectural structures that could leak data be flushed on trust level transitions. Also, it is important that trust levels do not concurrently execute on a single physical processor core.

This option controls whether microarchitectual structures are flushed on the kernel to user exit path, if possible. It may have a negative performance impact.

  • If set to true (the default), structures are flushed if the processor is vulnerable.
  • If set to false, no flush is executed on structures.

kernel.x86.pti.enable=<uint32_t>

Default: 0x2

Page table isolation configures user page tables to not have kernel text or data mapped. This may impact performance negatively. This is a mitigation for Meltdown (AKA CVE-2017-5754).

  • If set to 1, this force-enables page table isolation.
  • If set to 0, this force-disables page table isolation. This may be insecure.
  • If set to 2 or unset (the default), this enables page table isolation on CPUs vulnerable to Meltdown.

TODO(joshuaseaton): make this an enum instead of using magic integers.

kernel.x86.spec_store_bypass_disable=<bool>

Default: false

Spec-store-bypass (Spectre V4) is a speculative execution information leak vulnerability that affects many Intel and AMD x86 CPUs. It targets memory disambiguation hardware to infer the contents of recent stores. The attack only affects same-privilege-level, intra-process data.

This command line option controls whether a mitigation is enabled. The mitigation has negative performance impacts.

  • If true, the mitigation is enabled on CPUs that need it.
  • If false (the default), the mitigation is not enabled.

TODO: put something here