linux-kernel - [PATCH 4.14 68/71] bpf: fix 32-bit divide by zero

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Message-Id: <20180129123832.129423809@linuxfoundation.org>
Date:   Mon, 29 Jan 2018 13:57:36 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "ast@...nel.org, stable@...r.kernel.org, Daniel Borkmann" 
        <daniel@...earbox.net>,
        syzbot+48340bb518e88849e2e3@...kaller.appspotmail.com,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH 4.14 68/71] bpf: fix 32-bit divide by zero

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Alexei Starovoitov <ast@...nel.org>

[ upstream commit 68fda450a7df51cff9e5a4d4a4d9d0d5f2589153 ]

due to some JITs doing if (src_reg == 0) check in 64-bit mode
for div/mod operations mask upper 32-bits of src register
before doing the check

Fixes: 622582786c9e ("net: filter: x86: internal BPF JIT")
Fixes: 7a12b5031c6b ("sparc64: Add eBPF JIT.")
Reported-by: syzbot+48340bb518e88849e2e3@...kaller.appspotmail.com
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/bpf/verifier.c |   18 ++++++++++++++++++
 net/core/filter.c     |    4 ++++
 2 files changed, 22 insertions(+)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4304,6 +4304,24 @@ static int fixup_bpf_calls(struct bpf_ve
 	int i, cnt, delta = 0;
 
 	for (i = 0; i < insn_cnt; i++, insn++) {
+		if (insn->code == (BPF_ALU | BPF_MOD | BPF_X) ||
+		    insn->code == (BPF_ALU | BPF_DIV | BPF_X)) {
+			/* due to JIT bugs clear upper 32-bits of src register
+			 * before div/mod operation
+			 */
+			insn_buf[0] = BPF_MOV32_REG(insn->src_reg, insn->src_reg);
+			insn_buf[1] = *insn;
+			cnt = 2;
+			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
+			if (!new_prog)
+				return -ENOMEM;
+
+			delta    += cnt - 1;
+			env->prog = prog = new_prog;
+			insn      = new_prog->insnsi + i + delta;
+			continue;
+		}
+
 		if (insn->code != (BPF_JMP | BPF_CALL))
 			continue;
 
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -457,6 +457,10 @@ do_pass:
 			    convert_bpf_extensions(fp, &insn))
 				break;
 
+			if (fp->code == (BPF_ALU | BPF_DIV | BPF_X) ||
+			    fp->code == (BPF_ALU | BPF_MOD | BPF_X))
+				*insn++ = BPF_MOV32_REG(BPF_REG_X, BPF_REG_X);
+
 			*insn = BPF_RAW_INSN(fp->code, BPF_REG_A, BPF_REG_X, 0, fp->k);
 			break;