[PATCH] seccomp: Add pkru into seccomp_data

[Michael Sammler]

On 10/25/2018 11:12 AM, Florian Weimer wrote:
>> I understand your concern about exposing the number of protection keys
>> in the ABI. One idea would be to state, that the pkru field (which
>> should probably be renamed) contains an architecture specific value,
>> which could then be the PKRU on x86 and AMR (or another register) on
>> POWER. This new field should probably be extended to __u64 and the
>> reserved field removed.
> POWER also has proper read/write bit separation, not PKEY_DISABLE_ACCESS
> (disable read and write) and PKEY_DISABLE_WRITE like Intel.  It's
> currently translated by the kernel, but I really need a
> PKEY_DISABLE_READ bit in glibc to implement pkey_get in case the memory
> is write-only.
The idea here would be to simply provide the raw value of the register 
(PKRU on x86, AMR on POWER) to the BPF program and let the BPF program 
(or maybe a higher level library like libseccomp) deal with the 
complications of interpreting this architecture specific value (similar 
how the BPF program currently already has to deal with architecture 
specific system call numbers). If an architecture were to support more 
protection keys than fit into the field, the architecture specific value 
stored in the field might simply be the first protection keys. If there 
was interest, it would be possible to add more architecture specific 
fields to seccomp_data.
>> Another idea would be to not add a field in the seccomp_data
>> structure, but instead provide a new BPF instruction, which reads the
>> value of a specified protection key.
> I would prefer that if it's possible.  We should make sure that the bits
> are the same as those returned from pkey_get.  I have an implementation
> on POWER, but have yet to figure out the implications for 32-bit because
> I do not know the AMR register size there.
>
> Thanks,
> Florian
I have had a look at how BPF is implemented and it does not seem to be 
easy to just add an BPF instruction for seccomp since (as far as I 
understand) the code of the classical BPF (as used by seccomp) is shared 
with the code of eBPF, which is used in many parts of the kernel and 
there is at least one interpreter and one JIT compiler for BPF. But 
maybe someone with more experience than me can comment on how hard it 
would be to add an instruction to BPF.

- Michael