[PATCH bpf v2 1/2] bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension

From: Luke Nelson <lukenels@cs.washington.edu>
To: 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, "H. Peter Anvin" <hpa@zytor.com>,
	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
Subject: [PATCH bpf v2 1/2] bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension
Date: Wed, 22 Apr 2020 10:36:29 -0700	[thread overview]
Message-ID: <20200422173630.8351-1-luke.r.nels@gmail.com> (raw)

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@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.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:
-- 
2.17.1

