Re: [PATCH 10/17] prmem: documentation

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Nov 13, 2018 at 9:43 AM Nadav Amit <nadav.amit@gmail.com> wrote:
>
> From: Andy Lutomirski
> Sent: November 13, 2018 at 5:16:09 PM GMT
> > To: Igor Stoppa <igor.stoppa@gmail.com>
> > Cc: Kees Cook <keescook@chromium.org>, Peter Zijlstra <peterz@infradead.org>, Nadav Amit <nadav.amit@gmail.com>, Mimi Zohar <zohar@linux.vnet.ibm.com>, Matthew Wilcox <willy@infradead.org>, Dave Chinner <david@fromorbit.com>, James Morris <jmorris@namei.org>, Michal Hocko <mhocko@kernel.org>, Kernel Hardening <kernel-hardening@lists.openwall.com>, linux-integrity <linux-integrity@vger.kernel.org>, LSM List <linux-security-module@vger.kernel.org>, Igor Stoppa <igor.stoppa@huawei.com>, Dave Hansen <dave.hansen@linux.intel.com>, Jonathan Corbet <corbet@lwn.net>, Laura Abbott <labbott@redhat.com>, Randy Dunlap <rdunlap@infradead.org>, Mike Rapoport <rppt@linux.vnet.ibm.com>, open list:DOCUMENTATION <linux-doc@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>
> > Subject: Re: [PATCH 10/17] prmem: documentation
> >
> >
> > On Tue, Nov 13, 2018 at 6:25 AM Igor Stoppa <igor.stoppa@gmail.com> wrote:
> >> Hello,
> >> I've been studying v4 of the patch-set [1] that Nadav has been working on.
> >> Incidentally, I think it would be useful to cc also the
> >> security/hardening ml.
> >> The patch-set seems to be close to final, so I am resuming this discussion.
> >>
> >> On 30/10/2018 19:06, Andy Lutomirski wrote:
> >>
> >>> I support the addition of a rare-write mechanism to the upstream kernel.  And I think that there is only one sane way to implement it: using an mm_struct. That mm_struct, just like any sane mm_struct, should only differ from init_mm in that it has extra mappings in the *user* region.
> >>
> >> After reading the code, I see what you meant.
> >> I think I can work with it.
> >>
> >> But I have a couple of questions wrt the use of this mechanism, in the
> >> context of write rare.
> >>
> >>
> >> 1) mm_struct.
> >>
> >> Iiuc, the purpose of the patchset is mostly (only?) to patch kernel code
> >> (live patch?), which seems to happen sequentially and in a relatively
> >> standardized way, like replacing the NOPs specifically placed in the
> >> functions that need patching.
> >>
> >> This is a bit different from the more generic write-rare case, applied
> >> to data.
> >>
> >> As example, I have in mind a system where both IMA and SELinux are in use.
> >>
> >> In this system, a file is accessed for the first time.
> >>
> >> That would trigger 2 things:
> >> - evaluation of the SELinux rules and probably update of the AVC cache
> >> - IMA measurement and update of the measurements
> >>
> >> Both of them could be write protected, meaning that they would both have
> >> to be modified through the write rare mechanism.
> >>
> >> While the events, for 1 specific file, would be sequential, it's not
> >> difficult to imagine that multiple files could be accessed at the same time.
> >>
> >> If the update of the data structures in both IMA and SELinux must use
> >> the same mm_struct, that would have to be somehow regulated and it would
> >> introduce an unnecessary (imho) dependency.
> >>
> >> How about having one mm_struct for each writer (core or thread)?
> >
> > I don't think that helps anything.  I think the mm_struct used for
> > prmem (or rare_write or whatever you want to call it) should be
> > entirely abstracted away by an appropriate API, so neither SELinux nor
> > IMA need to be aware that there's an mm_struct involved.  It's also
> > entirely possible that some architectures won't even use an mm_struct
> > behind the scenes -- x86, for example, could have avoided it if there
> > were a kernel equivalent of PKRU.  Sadly, there isn't.
> >
> >> 2) Iiuc, the purpose of the 2 pages being remapped is that the target of
> >> the patch might spill across the page boundary, however if I deal with
> >> the modification of generic data, I shouldn't (shouldn't I?) assume that
> >> the data will not span across multiple pages.
> >
> > The reason for the particular architecture of text_poke() is to avoid
> > memory allocation to get it working.  i think that prmem/rare_write
> > should have each rare-writable kernel address map to a unique user
> > address, possibly just by offsetting everything by a constant.  For
> > rare_write, you don't actually need it to work as such until fairly
> > late in boot, since the rare_writable data will just be writable early
> > on.
> >
> >> If the data spans across multiple pages, in unknown amount, I suppose
> >> that I should not keep interrupts disabled for an unknown time, as it
> >> would hurt preemption.
> >>
> >> What I thought, in my initial patch-set, was to iterate over each page
> >> that must be written to, in a loop, re-enabling interrupts in-between
> >> iterations, to give pending interrupts a chance to be served.
> >>
> >> This would mean that the data being written to would not be consistent,
> >> but it's a problem that would have to be addressed anyways, since it can
> >> be still read by other cores, while the write is ongoing.
> >
> > This probably makes sense, except that enabling and disabling
> > interrupts means you also need to restore the original mm_struct (most
> > likely), which is slow.  I don't think there's a generic way to check
> > whether in interrupt is pending without turning interrupts on.
>
> I guess that enabling IRQs might break some hidden assumptions in the code,
> but is there a fundamental reason that IRQs need to be disabled? use_mm()
> got them enabled, although it is only suitable for kernel threads.
>

For general rare-writish stuff, I don't think we want IRQs running
with them mapped anywhere for write.  For AVC and IMA, I'm less sure.

--Andy