Re: [PATCH bpf-next v8 1/3] bpf: make common crypto API for TC/XDP programs

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 25/01/2024 01:10, Martin KaFai Lau wrote:
> On 1/15/24 2:08 PM, Vadim Fedorenko wrote:
>> +static int bpf_crypto_crypt(const struct bpf_crypto_ctx *ctx,
>> +                const struct bpf_dynptr_kern *src,
>> +                struct bpf_dynptr_kern *dst,
>> +                const struct bpf_dynptr_kern *siv,
>> +                bool decrypt)
>> +{
>> +    u32 src_len, dst_len, siv_len;
>> +    const u8 *psrc;
>> +    u8 *pdst, *piv;
>> +    int err;
>> +
>> +    if (ctx->type->get_flags(ctx->tfm) & CRYPTO_TFM_NEED_KEY)
> 
> nit. Does the indirect call get_flags() return different values?
> Should it be rejected earlier, e.g. in bpf_crypto_ctx_create()?

Well, that is the common pattern in crypto subsys to check flags.
But after looking at it second time, I think I have to refactor this
part. CRYPTO_TFM_NEED_KEY is set during tfm creation if algo requires
the key. And it's freed when the key setup is successful. As there is no
way bpf programs can modify tfm directly we can move this check to
bpf_crypto_ctx_create() to key setup part and avoid indirect call in 
this place.
> 
>> +        return -EINVAL;
>> +
>> +    if (__bpf_dynptr_is_rdonly(dst))
>> +        return -EINVAL;
>> +
>> +    siv_len = __bpf_dynptr_size(siv);
>> +    src_len = __bpf_dynptr_size(src);
>> +    dst_len = __bpf_dynptr_size(dst);
>> +    if (!src_len || !dst_len)
>> +        return -EINVAL;
>> +
>> +    if (siv_len != (ctx->type->ivsize(ctx->tfm) + 
>> ctx->type->statesize(ctx->tfm)))
> 
> Same here, two indirect calls per en/decrypt kfunc call. Does the return 
> value change?

I have to check the size of IV provided by the caller, and then to avoid
indirect calls I have to store these values somewhere in ctx. It gives a
direct access to these values to bpf programs, which can potentially
abuse them. Not sure if it's good to open such opportunity.

> 
>> +        return -EINVAL;
>> +
>> +    psrc = __bpf_dynptr_data(src, src_len);
>> +    if (!psrc)
>> +        return -EINVAL;
>> +    pdst = __bpf_dynptr_data_rw(dst, dst_len);
>> +    if (!pdst)
>> +        return -EINVAL;
>> +
>> +    piv = siv_len ? __bpf_dynptr_data_rw(siv, siv_len) : NULL;
>> +    if (siv_len && !piv)
>> +        return -EINVAL;
>> +
>> +    err = decrypt ? ctx->type->decrypt(ctx->tfm, psrc, pdst, src_len, 
>> piv)
>> +              : ctx->type->encrypt(ctx->tfm, psrc, pdst, src_len, piv);
>> +
>> +    return err;
>> +}
>