linux-kernel - Re: [PATCH] x86/speculation: Add document to describe Spectre and its mitigations

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Date:   Sun, 23 Dec 2018 15:11:51 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Jiri Kosina <jikos@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, stable@...r.kernel.org, daniel@...earbox.net
Subject: Re: [PATCH] x86/speculation: Add document to describe Spectre and
 its mitigations

On Fri, Dec 21, 2018 at 09:44:44AM -0800, Tim Chen wrote:
> +
> +4. Kernel sandbox attacking kernel
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The kernel has support for running user-supplied programs within the
> +kernel.  Specific rules (such as bounds checking) are enforced on these
> +programs by the kernel to ensure that they do not violate access controls.
> +
> +eBPF is a kernel sub-system that uses user-supplied program
> +to execute JITed untrusted byte code inside the kernel. eBPF is used
> +for manipulating and examining network packets, examining system call
> +parameters for sand boxes and other uses.
> +
> +A malicious local process could upload and trigger an malicious
> +eBPF script to the kernel, with the script attacking the kernel
> +using variant 1 or 2 and reading memory.

Above is not correct.
The exploit for var2 does not load bpf progs into kernel.
Instead the bpf interpreter is speculatively executing bpf prog
that was never loaded.
Hence CONFIG_BPF_JIT_ALWAYS_ON=y is necessary to make var2 harder
to exploit.
Same goes for other in kernel interpreters and state machines.

> +
> +Necessary Prerequisites:
> +1. Malicious local process
> +2. eBPF JIT enabled for unprivileged users, attacking kernel with secrets
> +on the same machine.

This is not quite correct either.
Var 1 could have been exploited with and without JIT.
Also above sounds like that var1 is still exploitable through bpf
which is not the case.