Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:8 as permitted sender) client-ip=2620:137:e000::3:8;
MIME-Version: 1.0
References: <20231120144642.591358648@infradead.org> <20231120154948.708762225@infradead.org>
 <20231122021817.ggym3biyfeksiplo@macbook-pro-49.dhcp.thefacebook.com> <20231122111517.GR8262@noisy.programming.kicks-ass.net>
In-Reply-To: <20231122111517.GR8262@noisy.programming.kicks-ass.net>
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Wed, 22 Nov 2023 16:43:05 -0800
Message-ID: <CAADnVQLhWFKcxno53zqGtuiWwUcw+TU8gB2eCBRPQC=2y5vrFw@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/cfi,bpf: Fix BPF JIT call
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        "David S. Miller" <davem@davemloft.net>,
        David Ahern <dsahern@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>,
        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        clang-built-linux <llvm@lists.linux.dev>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Joao Moreira <joao@overdrivepizza.com>,
        Mark Rutland <mark.rutland@arm.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Wed, Nov 22, 2023 at 3:15=E2=80=AFAM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
>
> To be very explicit, let me list all the various forms of function
> calls:
>
> Traditional:
>
> foo:
>   ... code here ...
>   ret
>
> direct caller:
>
>   call foo
>
> indirect caller:
>
>   lea foo(%rip), %r11
>   call *%r11
>
> IBT:
>
> foo:
>   endbr64
>   ... code here ...
>   ret
>
> direct caller:
>
>   call foo / call foo+4
>
> indirect caller:
>
>   lea foo(%rip), %r11
>   ...
>   call *%r11
>
>
> kCFI:
>
> __cfi_foo:
>   movl $0x12345678, %rax
>   (11 nops when CALL_PADDING)
> foo:
>   endbr64 (when also IBT)
>   ... code here ...
>   ret
>
> direct caller:
>
>   call foo / call foo+4
>
> indirect caller:
>
>   lea foo(%rip), %r11
>   ...
>   movl $(-0x12345678), %r10d
>   addl -15(%r11), %r10d (or -4 without CALL_PADDING)
>   je 1f
>   ud2
> 1:call *%r11
>
>
> FineIBT (builds as kCFI + CALL_PADDING + IBT + RETPOLINE and runtime
> patches things to look like):
>
> __cfi_foo:
>   endbr64
>   subl $0x12345678, %r10d
>   jz foo
>   ud2
>   nop
> foo:
>   osp nop3 (was endbr64)
>   ... code here ...
>   ret
>
> direct caller:
>
>   call foo / call foo+4
>
> indirect caller:
>
>   lea foo(%rip), %r11
>   ...
>   movl $0x12345678, %r10d
>   subl $16, %r11
>   nop4
>   call *%r11

Got it. That helps a lot!
You kind of have this comment scattered through arch/x86/kernel/alternative=
.c
but having it in one place like above would go a long way.
Could you please add it to arch/x86/net/bpf_jit_comp.c
or arch/x86/include/asm/cfi.h next to enum cfi_mode ?

> > I'm not sure doing cfi_bpf_hash check in JITed code is completely solvi=
ng the problem.
> > From bpf_dispatcher_*_func() calling into JITed will work,
> > but this emit_prologue() is doing the same job for all bpf progs.
> > Some bpf progs call each other directly and indirectly.
> > bpf_dispatcher_*_func() -> JITed_BPF_A -> JITed_BPF_B.
> > A into B can be a direct call (which cfi doesn't care about) and
> > indirect via emit_bpf_tail_call_indirect()->emit_indirect_jump().
> > Should we care about fineibt/kcfi there too?
>
> The way I understood the tail-call thing to work is that it jumps to
> bpf_prog + X86_TAIL_CALL_OFFSET, we already emit an extra ENDBR there to
> make this work.
>
> So the A -> B indirect call is otherwise unadornen and only needs ENDBR.
>
> Ideally that would use kCFI/FineIBT but since it also skips some of the
> setup, this gets to be non-trivial, so I've let this be as is.

I see. yeah. The setup is not trivial indeed. Keep as-is is fine.

> So the kCFI thing is 'new' but readily inspected by objdump or godbolt:
>
>   https://godbolt.org/z/sGe18z3ca
>
> (@Sami, that .Ltmp15 thing, I don't see that in the kernel, what
> compiler flag makes that go away?)

I also noticed this discrepancy. It doesn't seem to be used.
Looks weird to spend 8 bytes to store -sizeof(ud2)

> As to FineIBT, that has a big comment in arch/x86/kernel/alternative.c
> where I rewrite the kCFI thing into FineIBT. I can refer there to avoid
> duplicating comments, would that work?

Just the above comment somewhere would work.
I wouldn't worry about duplication. This is tricky stuff.
When gcc folks get around implementing kcfi they will find it useful too.