Re: [RFC 00/10] Process-local memory allocations for hiding KVM secrets

[Andy Lutomirski]

On Wed, Jun 12, 2019 at 1:27 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>
>
> > On Jun 12, 2019, at 12:55 PM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> >
> >> On 6/12/19 10:08 AM, Marius Hillenbrand wrote:
> >> This patch series proposes to introduce a region for what we call
> >> process-local memory into the kernel's virtual address space.
> >
> > It might be fun to cc some x86 folks on this series.  They might have
> > some relevant opinions. ;)
> >
> > A few high-level questions:
> >
> > Why go to all this trouble to hide guest state like registers if all the
> > guest data itself is still mapped?
> >
> > Where's the context-switching code?  Did I just miss it?
> >
> > We've discussed having per-cpu page tables where a given PGD is only in
> > use from one CPU at a time.  I *think* this scheme still works in such a
> > case, it just adds one more PGD entry that would have to context-switched.
>
> Fair warning: Linus is on record as absolutely hating this idea. He might change his mind, but itâs an uphill battle.

I looked at the patch, and it (sensibly) has nothing to do with
per-cpu PGDs.  So it's in great shape!

Seriously, though, here are some very high-level review comments:

Please don't call it "process local", since "process" is meaningless.
Call it "mm local" or something like that.

We already have a per-mm kernel mapping: the LDT.  So please nix all
the code that adds a new VA region, etc, except to the extent that
some of it consists of valid cleanups in and of itself.  Instead,
please refactor the LDT code (arch/x86/kernel/ldt.c, mainly) to make
it use a more general "mm local" address range, and then reuse the
same infrastructure for other fancy things.  The code that makes it
KASLR-able should be in its very own patch that applies *after* the
code that makes it all work so that, when the KASLR part causes a
crash, we can bisect it.

+ /*
+ * Faults in process-local memory may be caused by process-local
+ * addresses leaking into other contexts.
+ * tbd: warn and handle gracefully.
+ */
+ if (unlikely(fault_in_process_local(address))) {
+ pr_err("page fault in PROCLOCAL at %lx", address);
+ force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)address, current);
+ }
+

Huh?  Either it's an OOPS or you shouldn't print any special
debugging.  As it is, you're just blatantly leaking the address of the
mm-local range to malicious user programs.

Also, you should IMO consider using this mechanism for kmap_atomic().
Hi, Nadav!