linux-kernel - Re: [PATCH bpf v2 1/2] bpf, x86_32: Fix incorrect encoding in BPF

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Date:   Wed, 22 Apr 2020 23:08:40 -0700
From:   hpa@...or.com
To:     Luke Nelson <lukenels@...washington.edu>, bpf@...r.kernel.org
CC:     Brian Gerst <brgerst@...il.com>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>, Wang YanQing <udknight@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf v2 1/2] bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension

On April 22, 2020 10:36:29 AM PDT, Luke Nelson <lukenels@...washington.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. 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.
>
>This patch fixes the problem by instead emitting "xor dst_hi,dst_hi"
>to clear the upper 32 bits. This fixes the problem and is more
>efficient
>than using MOV to load a zero immediate.
>
>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. But the
>JIT should avoid emitting incorrect encodings regardless.
>
>Fixes: 03f5781be2c7b ("bpf, x86_32: add eBPF JIT compiler for ia32")
>Signed-off-by: Xi Wang <xi.wang@...il.com>
>Signed-off-by: Luke Nelson <luke.r.nels@...il.com>
>---
>v1 -> v2: Updated commit message to better reflect the bug.
>          (H. Peter Anvin and Brian Gerst)
>---
> arch/x86/net/bpf_jit_comp32.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/arch/x86/net/bpf_jit_comp32.c
>b/arch/x86/net/bpf_jit_comp32.c
>index 4d2a7a764602..cc9ad3892ea6 100644
>--- a/arch/x86/net/bpf_jit_comp32.c
>+++ b/arch/x86/net/bpf_jit_comp32.c
>@@ -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. Peter Anvin (Intel) <hpa@...or.com>
-- 
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