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Date:   Wed, 22 Apr 2020 23:08:40 -0700
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Subject: Re: [PATCH bpf v2 1/2] bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension
To:     Luke Nelson <lukenels@cs.washington.edu>, bpf@vger.kernel.org
CC:     Brian Gerst <brgerst@gmail.com>,
        Luke Nelson <luke.r.nels@gmail.com>,
        Xi Wang <xi.wang@gmail.com>, Wang YanQing <udknight@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
From:   hpa@zytor.com
Message-ID: <57BB877E-685A-4FC8-945C-3E1F30CF5926@zytor.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On April 22, 2020 10:36:29 AM PDT, Luke Nelson <lukenels@cs=2Ewashington=2E=
edu> wrote:
>The current JIT uses the following sequence to zero-extend into the
>upper 32 bits of the destination register for BPF_LDX BPF_{B,H,W},
>when the destination register is not on the stack:
>
>  EMIT3(0xC7, add_1reg(0xC0, dst_hi), 0);
>
>The problem is that C7 /0 encodes a MOV instruction that requires a
>4-byte
>immediate; the current code emits only 1 byte of the immediate=2E This
>means that the first 3 bytes of the next instruction will be treated as
>the rest of the immediate, breaking the stream of instructions=2E
>
>This patch fixes the problem by instead emitting "xor dst_hi,dst_hi"
>to clear the upper 32 bits=2E This fixes the problem and is more
>efficient
>than using MOV to load a zero immediate=2E
>
>This bug may not be currently triggerable as BPF_REG_AX is the only
>register not stored on the stack and the verifier uses it in a limited
>way, and the verifier implements a zero-extension optimization=2E But the
>JIT should avoid emitting incorrect encodings regardless=2E
>
>Fixes: 03f5781be2c7b ("bpf, x86_32: add eBPF JIT compiler for ia32")
>Signed-off-by: Xi Wang <xi=2Ewang@gmail=2Ecom>
>Signed-off-by: Luke Nelson <luke=2Er=2Enels@gmail=2Ecom>
>---
>v1 -> v2: Updated commit message to better reflect the bug=2E
>          (H=2E Peter Anvin and Brian Gerst)
>---
> arch/x86/net/bpf_jit_comp32=2Ec | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/arch/x86/net/bpf_jit_comp32=2Ec
>b/arch/x86/net/bpf_jit_comp32=2Ec
>index 4d2a7a764602=2E=2Ecc9ad3892ea6 100644
>--- a/arch/x86/net/bpf_jit_comp32=2Ec
>+++ b/arch/x86/net/bpf_jit_comp32=2Ec
>@@ -1854,7 +1854,9 @@ static int do_jit(struct bpf_prog *bpf_prog, int
>*addrs, u8 *image,
> 					      STACK_VAR(dst_hi));
> 					EMIT(0x0, 4);
> 				} else {
>-					EMIT3(0xC7, add_1reg(0xC0, dst_hi), 0);
>+					/* xor dst_hi,dst_hi */
>+					EMIT2(0x33,
>+					      add_2reg(0xC0, dst_hi, dst_hi));
> 				}
> 				break;
> 			case BPF_DW:

Reviewed-by: H=2E Peter Anvin (Intel) <hpa@zytor=2Ecom>
--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E