bpf: fix incorrect sign extension in check_alu_op()

From: Jann Horn <jannh@google.com>

[ Upstream commit 95a762e2 ]

Distinguish between
BPF_ALU64|BPF_MOV|BPF_K (load 32-bit immediate, sign-extended to 64-bit)
and BPF_ALU|BPF_MOV|BPF_K (load 32-bit immediate, zero-padded to 64-bit);
only perform sign extension in the first case.

Starting with v4.14, this is exploitable by unprivileged users as long as
the unprivileged_bpf_disabled sysctl isn't set.

Debian assigned CVE-2017-16995 for this issue.

v3:
 - add CVE number (Ben Hutchings)

Fixes: 48461135 ("bpf: allow access into map value arrays")
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/bpf/verifier.c

@@ -1790,10 +1790,17 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 			/* case: R = imm
 			 * remember the value we stored into this reg
 			 */
+			u64 imm;
+
+			if (BPF_CLASS(insn->code) == BPF_ALU64)
+				imm = insn->imm;
+			else
+				imm = (u32)insn->imm;
+
 			regs[insn->dst_reg].type = CONST_IMM;
-			regs[insn->dst_reg].imm = insn->imm;
-			regs[insn->dst_reg].max_value = insn->imm;
-			regs[insn->dst_reg].min_value = insn->imm;
+			regs[insn->dst_reg].imm = imm;
+			regs[insn->dst_reg].max_value = imm;
+			regs[insn->dst_reg].min_value = imm;
 		}
 
 	} else if (opcode > BPF_END) {