Re: [PATCH v5 11/17] arm64: docs: document pointer authentication

[Kees Cook]

On Fri, Oct 5, 2018 at 1:47 AM, Kristina Martsenko
<kristina.martsenko@xxxxxxx> wrote:
> From: Mark Rutland <mark.rutland@xxxxxxx>
>
> Now that we've added code to support pointer authentication, add some
> documentation so that people can figure out if/how to use it.
>
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> [kristina: update cpu-feature-registers.txt]
> Signed-off-by: Kristina Martsenko <kristina.martsenko@xxxxxxx>
> Cc: Andrew Jones <drjones@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Ramana Radhakrishnan <ramana.radhakrishnan@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> ---
>  Documentation/arm64/booting.txt                |  8 +++
>  Documentation/arm64/cpu-feature-registers.txt  |  4 ++
>  Documentation/arm64/elf_hwcaps.txt             |  5 ++
>  Documentation/arm64/pointer-authentication.txt | 84 ++++++++++++++++++++++++++
>  4 files changed, 101 insertions(+)
>  create mode 100644 Documentation/arm64/pointer-authentication.txt
>
> diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
> index 8d0df62c3fe0..8df9f4658d6f 100644
> --- a/Documentation/arm64/booting.txt
> +++ b/Documentation/arm64/booting.txt
> @@ -205,6 +205,14 @@ Before jumping into the kernel, the following conditions must be met:
>      ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b0.
>    - The DT or ACPI tables must describe a GICv2 interrupt controller.
>
> +  For CPUs with pointer authentication functionality:
> +  - If EL3 is present:
> +    SCR_EL3.APK (bit 16) must be initialised to 0b1
> +    SCR_EL3.API (bit 17) must be initialised to 0b1
> +  - If the kernel is entered at EL1:
> +    HCR_EL2.APK (bit 40) must be initialised to 0b1
> +    HCR_EL2.API (bit 41) must be initialised to 0b1
> +
>  The requirements described above for CPU mode, caches, MMUs, architected
>  timers, coherency and system registers apply to all CPUs.  All CPUs must
>  enter the kernel in the same exception level.
> diff --git a/Documentation/arm64/cpu-feature-registers.txt b/Documentation/arm64/cpu-feature-registers.txt
> index 7964f03846b1..b165677ffab9 100644
> --- a/Documentation/arm64/cpu-feature-registers.txt
> +++ b/Documentation/arm64/cpu-feature-registers.txt
> @@ -190,6 +190,10 @@ infrastructure:
>       |--------------------------------------------------|
>       | JSCVT                        | [15-12] |    y    |
>       |--------------------------------------------------|
> +     | API                          | [11-8]  |    y    |
> +     |--------------------------------------------------|
> +     | APA                          | [7-4]   |    y    |
> +     |--------------------------------------------------|
>       | DPB                          | [3-0]   |    y    |
>       x--------------------------------------------------x
>
> diff --git a/Documentation/arm64/elf_hwcaps.txt b/Documentation/arm64/elf_hwcaps.txt
> index d6aff2c5e9e2..95509a7b0ffe 100644
> --- a/Documentation/arm64/elf_hwcaps.txt
> +++ b/Documentation/arm64/elf_hwcaps.txt
> @@ -178,3 +178,8 @@ HWCAP_ILRCPC
>  HWCAP_FLAGM
>
>      Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0001.
> +
> +HWCAP_APIA
> +
> +    EL0 AddPac and Auth functionality using APIAKey_EL1 is enabled, as
> +    described by Documentation/arm64/pointer-authentication.txt.
> diff --git a/Documentation/arm64/pointer-authentication.txt b/Documentation/arm64/pointer-authentication.txt
> new file mode 100644
> index 000000000000..8a9cb5713770
> --- /dev/null
> +++ b/Documentation/arm64/pointer-authentication.txt
> @@ -0,0 +1,84 @@
> +Pointer authentication in AArch64 Linux
> +=======================================
> +
> +Author: Mark Rutland <mark.rutland@xxxxxxx>
> +Date: 2017-07-19
> +
> +This document briefly describes the provision of pointer authentication
> +functionality in AArch64 Linux.
> +
> +
> +Architecture overview
> +---------------------
> +
> +The ARMv8.3 Pointer Authentication extension adds primitives that can be
> +used to mitigate certain classes of attack where an attacker can corrupt
> +the contents of some memory (e.g. the stack).
> +
> +The extension uses a Pointer Authentication Code (PAC) to determine
> +whether pointers have been modified unexpectedly. A PAC is derived from
> +a pointer, another value (such as the stack pointer), and a secret key
> +held in system registers.
> +
> +The extension adds instructions to insert a valid PAC into a pointer,
> +and to verify/remove the PAC from a pointer. The PAC occupies a number
> +of high-order bits of the pointer, which varies dependent on the
> +configured virtual address size and whether pointer tagging is in use.
> +
> +A subset of these instructions have been allocated from the HINT
> +encoding space. In the absence of the extension (or when disabled),
> +these instructions behave as NOPs. Applications and libraries using
> +these instructions operate correctly regardless of the presence of the
> +extension.
> +
> +
> +Basic support
> +-------------
> +
> +When CONFIG_ARM64_PTR_AUTH is selected, and relevant HW support is
> +present, the kernel will assign a random APIAKey value to each process
> +at exec*() time. This key is shared by all threads within the process,
> +and the key is preserved across fork(). Presence of functionality using
> +APIAKey is advertised via HWCAP_APIA.

It might be useful to include documentation here on how many bits of
the address are being used for the PAC bits (I'm assuming it's 7?)

-Kees

-- 
Kees Cook
Pixel Security