Re: [PATCH v4 6/6] Documentation: Add L1D flushing Documentation

[Kees Cook]

On Fri, Apr 24, 2020 at 12:01:25AM +1000, Balbir Singh wrote:
> Add documentation of l1d flushing, explain the need for the
> feature and how it can be used.
> 
> Signed-off-by: Balbir Singh <sblbir@xxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  Documentation/admin-guide/hw-vuln/index.rst   |  1 +
>  .../admin-guide/hw-vuln/l1d_flush.rst         | 40 +++++++++++++++++++
>  2 files changed, 41 insertions(+)
>  create mode 100644 Documentation/admin-guide/hw-vuln/l1d_flush.rst
> 
> diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst
> index 0795e3c2643f..35633b299d45 100644
> --- a/Documentation/admin-guide/hw-vuln/index.rst
> +++ b/Documentation/admin-guide/hw-vuln/index.rst
> @@ -14,3 +14,4 @@ are configurable at compile, boot or run time.
>     mds
>     tsx_async_abort
>     multihit.rst
> +   l1d_flush
> diff --git a/Documentation/admin-guide/hw-vuln/l1d_flush.rst b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
> new file mode 100644
> index 000000000000..7d515b8c29f1
> --- /dev/null
> +++ b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
> @@ -0,0 +1,40 @@
> +L1D Flushing for the paranoid
> +=============================
> +
> +With an increasing number of vulnerabilities being reported around data
> +leaks from L1D, a new user space mechanism to flush the L1D cache on
> +context switch is added to the kernel. This should help address
> +CVE-2020-0550 and for paranoid applications, keep them safe from any
> +yet to be discovered vulnerabilities, related to leaks from the L1D
> +cache.
> +
> +Tasks can opt in to this mechanism by using a prctl (implemented only
> +for x86 at the moment).
> +
> +Related CVES
> +------------
> +At the present moment, the following CVEs can be addressed by this
> +mechanism
> +
> +    =============       ========================     ==================
> +    CVE-2020-0550       Improper Data Forwarding     OS related aspects
> +    =============       ========================     ==================
> +
> +Usage Guidelines
> +----------------
> +Applications can call ``prctl(2)`` with one of these two arguments
> +
> +1. PR_SET_L1D_FLUSH - flush the L1D cache on context switch (out)
> +2. PR_GET_L1D_FLUSH - get the current state of the L1D cache flush, returns 1
> +   if set and 0 if not set.
> +
> +**NOTE**: The feature is disabled by default, applications to need to specifically
> +opt into the feature to enable it.
> +
> +Mitigation
> +----------
> +When PR_SET_L1D_FLUSH is enabled for a task, on switching tasks (when
> +the address space changes), a flush of the L1D cache is performed for
> +the task when it leaves the CPU. If the underlying CPU supports L1D
> +flushing in hardware, the hardware mechanism is used, otherwise a software
> +fallback, similar to the mechanism used by L1TF is used.
> -- 
> 2.17.1
> 

-- 
Kees Cook