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

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Message-ID: <20190109011130.wrsrcaly2mgnou3k@ast-mbp>
Date:   Tue, 8 Jan 2019 17:11:32 -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,
        davem@...emloft.net
Subject: Re: [PATCH] x86/speculation: Add document to describe Spectre and
 its mitigations

On Tue, Jan 08, 2019 at 01:12:45PM -0800, Tim Chen wrote:
> On 12/23/18 3:11 PM, Alexei Starovoitov wrote:
> > 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.
> > 
> 
> Alexi,
> 
> Do you have any suggestions on how to rewrite this two paragraphs?  You
> are probably the best person to update content for this section.

how about moving bpf bits out of this doc and placing them under Documentation/bpf/ ?
We can create bpf_security.rst there with specdown mitigations, best practices,
useful sysctl and config knobs, etc.