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Date: Fri, 24 Apr 2020 17:15:23 -0700 From: Alexei Starovoitov <alexei.starovoitov@...il.com> To: Luke Nelson <lukenels@...washington.edu> Cc: bpf <bpf@...r.kernel.org>, 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 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, 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>, Network Development <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH bpf v2 1/2] bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension On Wed, Apr 22, 2020 at 10:36 AM 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> Applied. Thanks
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