On Mon, Dec 6, 2021 at 3:22 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> Tag the return type of {per, this}_cpu_ptr with RDONLY_MEM. The
> returned value of this pair of helpers is kernel object, which
> can not be updated by bpf programs. Previously these two helpers
> return PTR_OT_MEM for kernel objects of scalar type, which allows
> one to directly modify the memory. Now with RDONLY_MEM tagging,
> the verifier will reject programs that writes into RDONLY_MEM.
>
> Fixes: 63d9b80dcf2c ("bpf: Introduce bpf_this_cpu_ptr()")
> Fixes: eaa6bcb71ef6 ("bpf: Introduce bpf_per_cpu_ptr()")
> Fixes: 4976b718c355 ("bpf: Introduce pseudo_btf_id")
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---
> kernel/bpf/helpers.c | 4 ++--
> kernel/bpf/verifier.c | 33 ++++++++++++++++++++++++++++-----
> 2 files changed, 30 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 293d9314ec7f..a5e349c9d3e3 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -667,7 +667,7 @@ BPF_CALL_2(bpf_per_cpu_ptr, const void *, ptr, u32, cpu)
> const struct bpf_func_proto bpf_per_cpu_ptr_proto = {
> .func = bpf_per_cpu_ptr,
> .gpl_only = false,
> - .ret_type = RET_PTR_TO_MEM_OR_BTF_ID | PTR_MAYBE_NULL,
> + .ret_type = RET_PTR_TO_MEM_OR_BTF_ID | PTR_MAYBE_NULL | MEM_RDONLY,
> .arg1_type = ARG_PTR_TO_PERCPU_BTF_ID,
> .arg2_type = ARG_ANYTHING,
> };
> @@ -680,7 +680,7 @@ BPF_CALL_1(bpf_this_cpu_ptr, const void *, percpu_ptr)
> const struct bpf_func_proto bpf_this_cpu_ptr_proto = {
> .func = bpf_this_cpu_ptr,
> .gpl_only = false,
> - .ret_type = RET_PTR_TO_MEM_OR_BTF_ID,
> + .ret_type = RET_PTR_TO_MEM_OR_BTF_ID | MEM_RDONLY,
> .arg1_type = ARG_PTR_TO_PERCPU_BTF_ID,
> };
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index f8b804918c35..44af65f07a82 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4296,16 +4296,32 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
> mark_reg_unknown(env, regs, value_regno);
> }
> }
> - } else if (reg->type == PTR_TO_MEM) {
> + } else if (base_type(reg->type) == PTR_TO_MEM) {
> + bool rdonly_mem = type_is_rdonly_mem(reg->type);
> +
> + if (type_may_be_null(reg->type)) {
> + verbose(env, "R%d invalid mem access '%s'\n", regno,
> + reg_type_str(reg->type));
see, here you'll get "invalid mem access 'ptr_to_mem'" while it's
actually ptr_to_mem_or_null. Like verifier logs are not hard enough to
follow, now they will be also misleading.
> + return -EACCES;
> + }
> +
> + if (t == BPF_WRITE && rdonly_mem) {
> + verbose(env, "R%d cannot write into rdonly %s\n",
> + regno, reg_type_str(reg->type));
> + return -EACCES;
> + }
> +
> if (t == BPF_WRITE && value_regno >= 0 &&
> is_pointer_value(env, value_regno)) {
> verbose(env, "R%d leaks addr into mem\n", value_regno);
> return -EACCES;
> }
> +
> err = check_mem_region_access(env, regno, off, size,
> reg->mem_size, false);
> - if (!err && t == BPF_READ && value_regno >= 0)
> - mark_reg_unknown(env, regs, value_regno);
> + if (!err && value_regno >= 0)
> + if (t == BPF_READ || rdonly_mem)
why two nested ifs for one condition?
> + mark_reg_unknown(env, regs, value_regno);
> } else if (reg->type == PTR_TO_CTX) {
> enum bpf_reg_type reg_type = SCALAR_VALUE;
> struct btf *btf = NULL;
[...]
