[PATCH 10/17] prmem: documentation

[Andy Lutomirski]

On Tue, Nov 13, 2018 at 6:25 AM Igor Stoppa <igor.stoppa at 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.