linux-kernel - Re: TLB flushes on fixmap changes

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Date:   Mon, 27 Aug 2018 11:55:00 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     mhiramat@...nel.org, Kees Cook <keescook@...omium.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>, jkosina@...e.cz,
        Will Deacon <will.deacon@....com>, benh@....ibm.com,
        npiggin@...il.com, "the arch/x86 maintainers" <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        Adin Scannell <ascannell@...gle.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "David S. Miller" <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: TLB flushes on fixmap changes

On Mon, Aug 27, 2018 at 10:13 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Aug 27, 2018 at 12:03:05PM +0900, Masami Hiramatsu wrote:
> > On Sun, 26 Aug 2018 11:09:58 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
>
> > > FWIW, before text_poke_bp(), text_poke() would only be used from
> > > stop_machine, so all the other CPUs would be stuck busy-waiting with
> > > IRQs disabled. These days, yeah, that's lots more dodgy, but yes
> > > text_mutex should be serializing all that.
> >
> > I'm still not sure that speculative page-table walk can be done
> > over the mutex. Also, if the fixmap area is for aliasing
> > pages (which always mapped to memory), what kind of
> > security issue can happen?
>
> So suppose CPU-A is doing the text_poke (let's say through text_poke_bp,
> such that other CPUs get to continue with whatever they're doing).
>
> While at that point, CPU-B gets an interrupt, and the CPU's
> branch-trace-buffer for the IRET points to / near our fixmap. Then the
> CPU could do a speculative TLB fill based on the BTB value, either
> directly or indirectly (through speculative driven fault-ahead) of
> whatever is in te fixmap at the time.

Worse: The way academics have been defeating KASLR for a while is
based on TLB fills for kernel addresses, triggered from userspace.
Quoting https://www.ieee-security.org/TC/SP2013/papers/4977a191.pdf :

| Additionally, even if a permission error occurs, this still allows to
| launch address translations and, hence, generate valid TLB entries
| by accessing privileged kernel space memory from user mode.

This was actually part of the original motivation for KAISER/KPTI.
Quoting https://gruss.cc/files/kaiser.pdf :

| Modern operating system kernels employ address space layout
| randomization (ASLR) to prevent control-flow hijacking attacks and
| code-injection attacks. While kernel security relies fundamentally
on preventing
| access to address information, recent attacks have shown that the
| hardware directly leaks this information.

I believe that PTI probably prevents this way of directly triggering
TLB fills for now (under the assumption that hyperthreads with equal
CR3 don't share TLB entries), but I would still assume that an
attacker can probably trigger TLB fills for arbitrary addresses
anytime. And at some point in the future, I believe people would
probably like to be able to disable PTI again?

> Then CPU-A completes the text_poke and only does a local TLB invalidate
> on CPU-A, leaving CPU-B with an active translation.
>
> *FAIL*