From 5fa9a98fb10380e48a398998cd36a85e4ef711d6 Mon Sep 17 00:00:00 2001 From: Luke Nelson Date: Wed, 22 Apr 2020 10:36:29 -0700 Subject: [PATCH] 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: 03f5781be2c7b ("bpf, x86_32: add eBPF JIT compiler for ia32") Signed-off-by: Xi Wang Signed-off-by: Luke Nelson Signed-off-by: Alexei Starovoitov Reviewed-by: H. Peter Anvin (Intel) Acked-by: Wang YanQing Link: https://lore.kernel.org/bpf/20200422173630.8351-1-luke.r.nels@gmail.com --- 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 4d2a7a7..cc9ad38 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: -- 2.7.4