linux-kernel - [PATCH 5.6 061/106] bpf, x86_32: Fix incorrect encoding in BPF

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Message-Id: <20200501131550.891106146@linuxfoundation.org>
Date:   Fri,  1 May 2020 15:23:34 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Xi Wang <xi.wang@...il.com>,
        Luke Nelson <luke.r.nels@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        "H. Peter Anvin (Intel)" <hpa@...or.com>,
        Wang YanQing <udknight@...il.com>
Subject: [PATCH 5.6 061/106] bpf, x86_32: Fix incorrect encoding in BPF_LDX zero-extension

From: Luke Nelson <lukenels@...washington.edu>

commit 5fa9a98fb10380e48a398998cd36a85e4ef711d6 upstream.

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>
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Reviewed-by: H. Peter Anvin (Intel) <hpa@...or.com>
Acked-by: Wang YanQing <udknight@...il.com>
Link: https://lore.kernel.org/bpf/20200422173630.8351-1-luke.r.nels@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/net/bpf_jit_comp32.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- 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_p
 					      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: