From: Jann Horn jannh@google.com
Reject programs that compute wildly out-of-bounds stack pointers. Otherwise, pointers can be computed with an offset that doesn't fit into an `int`, causing security issues in the stack memory access check (as well as signed integer overflow during offset addition).
This is a fix specifically for the v4.9 stable tree because the mainline code looks very different at this point.
Fixes: 7bca0a9702edf ("bpf: enhance verifier to understand stack pointer arithmetic") Signed-off-by: Jann Horn jannh@google.com Acked-by: Daniel Borkmann daniel@iogearbox.net --- kernel/bpf/verifier.c | 22 ++++++++++++++++++++-- 1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 2f19c94..b03af36 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1861,10 +1861,28 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn) ((BPF_SRC(insn->code) == BPF_X && regs[insn->src_reg].type == CONST_IMM) || BPF_SRC(insn->code) == BPF_K)) { - if (BPF_SRC(insn->code) == BPF_X) + if (BPF_SRC(insn->code) == BPF_X) { + /* check in case the register contains a big + * 64-bit value + */ + if (regs[insn->src_reg].imm < -MAX_BPF_STACK || + regs[insn->src_reg].imm > MAX_BPF_STACK) { + verbose("R%d value too big in R%d pointer arithmetic\n", + insn->src_reg, insn->dst_reg); + return -EACCES; + } dst_reg->imm += regs[insn->src_reg].imm; - else + } else { + /* safe against overflow: addition of 32-bit + * numbers in 64-bit representation + */ dst_reg->imm += insn->imm; + } + if (dst_reg->imm > 0 || dst_reg->imm < -MAX_BPF_STACK) { + verbose("R%d out-of-bounds pointer arithmetic\n", + insn->dst_reg); + return -EACCES; + } return 0; } else if (opcode == BPF_ADD && BPF_CLASS(insn->code) == BPF_ALU64 &&