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

[Kristina Martsenko]

On 05/10/2018 10:04, Ramana Radhakrishnan wrote:
> On 05/10/2018 09:47, Kristina Martsenko 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.
> 
> s/pointer tagging/top byte ignore unless that's the terminology in the
> rest of the kernel documentation ?

The rest of the kernel documentation calls them "tagged pointers", and
doesn't use "top byte ignore", for example
Documentation/arm64/tagged-pointers.txt:
https://elixir.bootlin.com/linux/latest/source/Documentation/arm64/tagged-pointers.txt

> 
>> +
>> +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.
>> +
>> +Recent versions of GCC can compile code with APIAKey-based return
>> +address protection when passed the -msign-return-address option. This
>> +uses instructions in the HINT space, and such code can run on systems
>> +without the pointer authentication extension.
> 
> Just a clarification.
> 
> This uses instructions in the hint space for architecture levels less
> than armv8.3-a by default. If folks use -march=armv8.3-a you will start
> seeing the combined forms of retaa appear.

I'll amend this to:

"This uses instructions in the HINT space (unless -march=armv8.3-a or
higher is also passed), and such code can run on systems without the
pointer authentication extension."

> 
>> +
>> +The remaining instruction and data keys (APIBKey, APDAKey, APDBKey) are
>> +reserved for future use, and instructions using these keys must not be
>> +used by software until a purpose and scope for their use has been
>> +decided. To enable future software using these keys to function on
>> +contemporary kernels, where possible, instructions using these keys are
>> +made to behave as NOPs.
>> +
>> +The generic key (APGAKey) is currently unsupported. Instructions using
>> +the generic key must not be used by software.
>> +
>> +
>> +Debugging
>> +---------
>> +
>> +When CONFIG_ARM64_PTR_AUTH is selected, and relevant HW support is
>> +present, the kernel will expose the position of TTBR0 PAC bits in the
>> +NT_ARM_PAC_MASK regset (struct user_pac_mask), which userspace can
>> +acqure via PTRACE_GETREGSET.
>> +
>> +Separate masks are exposed for data pointers and instruction pointers,
>> +as the set of PAC bits can vary between the two. Debuggers should not
>> +expect that HWCAP_APIA implies the presence (or non-presence) of this
>> +regset -- in future the kernel may support the use of APIBKey, APDAKey,
>> +and/or APBAKey, even in the absence of APIAKey.
>> +
>> +Note that the masks apply to TTBR0 addresses, and are not valid to apply
>> +to TTBR1 addresses (e.g. kernel pointers).
>> +
>> +
>> +Virtualization
>> +--------------
>> +
>> +Pointer authentication is not currently supported in KVM guests. KVM
>> +will mask the feature bits from ID_AA64ISAR1_EL1, and attempted use of
>> +the feature will result in an UNDEFINED exception being injected into
>> +the guest.
> 
> However applications using instructions from the hint space will
> continue to work albeit without any protection (as they would just be
> nops) ?

Mostly, yes. If the guest leaves SCTLR_EL1.EnIA unset (and
EnIB/EnDA/EnDB), then PAC* and AUT* instructions in the HINT space will
execute as NOPs. If the guest sets EnIA, then PAC*/AUT* instructions
will trap and KVM will inject an "Unknown reason" exception into the
guest (which will cause a Linux guest to send a SIGILL to the application).

In the latter case we could instead pretend the instruction was a NOP
and not inject an exception, but trapping twice per every function would
probably be terrible for performance. The guest shouldn't be setting
EnIA anyway if ID_AA64ISAR1_EL1 reports that pointer authentication is
not present (because KVM has hidden it).

The other special case is the XPACLRI instruction, which is also in the
HINT space. Currently it will trap and KVM will inject an exception into
the guest. We should probably change this to NOP instead, as that's what
applications will expect. Unfortunately there is no EnIA-like control to
make it NOP.

One option is for KVM to pretend the instruction was a NOP and return to
the guest. But if XPACLRI gets executed frequently, then the constant
trapping might hurt performance. I don't know how frequently it might
get used, as I don't know of any applications currently using it. From
what I understand, it may be used by userspace stack unwinders.

(Also worth noting - as far as I can tell there is no easy way for KVM
to know which pointer authentication instruction caused the trap, so we
may have to do something unusual like use "at s12e1r" to read guest
memory and check for XPACLRI.)

The other option is to turn off trapping entirely. However then on a
big.LITTLE system with mismatched pointer authentication support
instructions will work intermittently on some CPUs but not others.

Thoughts?

> 
> Reviewed-by: Ramana Radhakrishnan  <ramana.radhakrishnan@xxxxxxx>

Thanks!

Kristina