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Date: Wed, 26 Jun 2019 10:14:50 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Florian Weimer <fweimer@...hat.com>
Cc: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
Linux API <linux-api@...r.kernel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-x86_64@...r.kernel.org,
linux-arch <linux-arch@...r.kernel.org>, Kees Cook <keescook@...omium.org>,
"Carlos O'Donell" <carlos@...hat.com>, X86 ML <x86@...nel.org>
Subject: Re: Detecting the availability of VSYSCALL
On Wed, Jun 26, 2019 at 10:04 AM Florian Weimer <fweimer@...hat.com> wrote:
>
> * Andy Lutomirski:
>
> > On Wed, Jun 26, 2019 at 9:45 AM Florian Weimer <fweimer@...hat.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.
Ugh. I replied there. I don't consider any of that to have much
bearing on what we do for vsyscalls. That being said, the
PT_GNU_PROPERTY thing sounds like maybe we could use it for a bit
saying "no vsyscalls needed".
>
> The thread is:
>
> Subject: Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an ELF file
>
> <87blyu7ubf.fsf@...enburg2.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.
We do have that, but I don't think we have it for personality. The
whole personality mechanism scares me a bit due to a lack of this type
of thing, and I'd want to review it carefully before adding a new
personality bit.
--Andy
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