Re: Detecting the availability of VSYSCALL

[Florian Weimer <fweimer@redhat.com>]

* Andy Lutomirski:

> On Wed, Jun 26, 2019 at 9:45 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * Andy Lutomirski:
>>
>> > Can’t an ELF note be done with some more or less ordinary asm such
>> > that any link editor will insert it correctly?
>>
>> We've just been over this for the CET enablement.  ELF PT_NOTE parsing
>> was rejected there.
>
> No one told me this.  Unless I missed something, the latest kernel
> patches still had PT_NOTE parsing.  Can you point me at an
> enlightening thread or explain what happened?

The ABI was changed rather late, and PT_GNU_PROPERTY has been added.
But this is okay because the kernel only looks at the dynamic loader,
which we can update fairly easily.

The thread is:

Subject: Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an ELF file

<87blyu7ubf.fsf@oldenburg2.str.redhat.com> is a message reference in it.

>> > The problem with a personality flag is that it needs to have some kind
>> > of sensible behavior for setuid programs, and getting that right in a
>> > way that doesn’t scream “exploit me” while preserving useful
>> > compatibility may be tricky.
>>
>> Are restrictive personality flags still a problem with user namespaces?
>> I think it would be fine to restrict this one to CAP_SYS_ADMIN.
>
> We could possibly get away with this, but now we're introducing a
> whole new mechanism.  I'd rather just add proper per-namespace
> sysctls, but this is a pretty big hammer.

Oh, I wasn't aware of that.  I thought that this already existed in some
form, e.g. prctl with PR_SET_SECCOMP requiring CAP_SYS_ADMIN unless
PR_SET_NO_NEW_PRIVS was active as well.

Thanks,
Florian