netdev - [PATCH] bpf: fix shift overflow in ___bpf_prog

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Date:   Tue, 8 Jan 2019 14:16:59 +0800
From:   ZhangXiaoxu <zhangxiaoxu5@...wei.com>
To:     <ast@...nel.org>, <netdev@...r.kernel.org>
Subject: [PATCH] bpf: fix shift overflow in ___bpf_prog_run

From: Zhang Xiaoxu <zhangxiaoxu5@...wei.com>

There is a UBSAN bug as blew:
UBSAN: Undefined behaviour in kernel/bpf/core.c:1055:2
shift exponent 511 is too large for 32-bit type 'unsigned int'

Reproduce program:
	#include <errno.h>
	#include <stddef.h>
	#include <stdio.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <linux/audit.h>
	#include <linux/filter.h>
	#include <linux/seccomp.h>
	#include <sys/prctl.h>
	#include <sys/syscall.h>

	int main() {
		struct sock_filter sock_filter[3] = {
			BPF_JUMP(BPF_LDX|BPF_IMM, 0x1ff, 0x2, 0xfffffffffffffffd),
			BPF_JUMP(BPF_ALU|BPF_LSH|BPF_X, 0x0, 0x506, 0x401),
			BPF_JUMP(BPF_RET|BPF_K, 0x0, 0x0, SECCOMP_RET_KILL)
		};

		struct sock_fprog sock_fprog= {
			.len = 3,
			.filter = &sock_filter,
		};

		int ret = syscall(__NR_seccomp, SECCOMP_SET_MODE_FILTER, 0, &sock_fprog);
		printf("%d\n", ret);

		return 0;
	}

Make sure the right operand not greater than or equal to the
width of the promoted left operand when do shift operation.

Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@...wei.com>
---
 kernel/bpf/core.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 503d421..7635557 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -683,11 +683,26 @@ static unsigned int __bpf_prog_run(void *ctx, const struct bpf_insn *insn)
 	ALU(SUB,  -)
 	ALU(AND,  &)
 	ALU(OR,   |)
-	ALU(LSH, <<)
-	ALU(RSH, >>)
 	ALU(XOR,  ^)
 	ALU(MUL,  *)
 #undef ALU
+#define ALU_SHIFT(OPCODE, OP)			\
+	ALU64_##OPCODE##_X: 		\
+		DST = DST OP (SRC & 0x3F);	\
+		CONT;			\
+	ALU_##OPCODE##_X:		\
+		DST = (u32) DST OP (SRC & 0x1F);\
+		CONT;			\
+	ALU64_##OPCODE##_K: 		\
+		DST = DST OP (IMM & 0x3F);	\
+		CONT;			\
+	ALU_##OPCODE##_K:		\
+		DST = (u32) DST OP (IMM & 0x1F);\
+		CONT;
+
+	ALU_SHIFT(LSH, <<)
+	ALU_SHIFT(RSH, >>)
+#undef ALU_SHIFT
 	ALU_NEG:
 		DST = (u32) -DST;
 		CONT;
-- 
2.7.4