linux-kernel - Re: [PATCH 0/2] x86/bpf: Fix FineIBT vs eBPF

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Date:   Wed, 22 Nov 2023 11:17:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, davem@...emloft.net, dsahern@...nel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yonghong.song@...ux.dev,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, Arnd Bergmann <arnd@...db.de>,
        samitolvanen@...gle.com, keescook@...omium.org, nathan@...nel.org,
        ndesaulniers@...gle.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-arch@...r.kernel.org,
        llvm@...ts.linux.dev, jpoimboe@...nel.org, joao@...rdrivepizza.com,
        mark.rutland@....com
Subject: Re: [PATCH 0/2] x86/bpf: Fix FineIBT vs eBPF

On Tue, Nov 21, 2023 at 05:41:07PM -0800, Alexei Starovoitov wrote:
> On Mon, Nov 20, 2023 at 03:46:42PM +0100, Peter Zijlstra wrote:
> > Hi!
> > 
> > There's a problem with FineIBT and eBPF using __nocfi when
> > CONFIG_BPF_JIT_ALWAYS_ON=n, in which case the __nocfi indirect call can target
> > a normal function like __bpf_prog_run32().
> 
> The lack (or partially broken) cfi in the kernel built with
> CONFIG_BPF_JIT_ALWAYS_ON=n is probably the last of people security concerns.
> We introduced CONFIG_BPF_JIT_ALWAYS_ON=y to remove the interpreter,
> since mere presence of _any_ interpreter in the kernel (bpf and any other)
> is an attack vector. As it was demonstrated during spectre days an interpreter
> sitting in executable part of vmlinux .text tremendously helps to craft
> a speculative execution exploit.

Oh, no argument there. I always have JIT_ALWAYS_ON=y (when I have BPF at
all) which is why it took me so long to actually trip over this.

This was a test script systematically build/boot a bunch of configs and
going unexpectedly *splat*.

But it was a good excuse to spend time fixing it.