bpf: fix incorrect sign extension in check_alu_op()

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: 484611357c19 ("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>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name bpf-fix-incorrect-sign-extension-in-check_alu_op.patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b03af36b8e876c45d9df9f277bf6605a8bb4dbe5..8b1ebe4c6abaab7641c39166aaeb59937776010f 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/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) {