bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension

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: 03f5781be2 ("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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Acked-by: Wang YanQing <udknight@gmail.com>
Link: https://lore.kernel.org/bpf/20200422173630.8351-1-luke.r.nels@gmail.com

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: