Date: Wed, 1 Nov 2023 22:50:54 +0000
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: Martin KaFai Lau <martin.lau@...ux.dev>,
 "David S. Miller" <davem@...emloft.net>,
 Herbert Xu <herbert@...dor.apana.org.au>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
 linux-crypto@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
 Mykola Lysenko <mykolal@...com>, Vadim Fedorenko <vadfed@...a.com>
Subject: Re: [PATCH bpf-next v3 1/2] bpf: add skcipher API support to TC/XDP
 programs

On 01/11/2023 21:49, Martin KaFai Lau wrote:
> On 10/31/23 6:48 AM, Vadim Fedorenko wrote:
>> --- /dev/null
>> +++ b/kernel/bpf/crypto.c
>> @@ -0,0 +1,280 @@
>> +// SPDX-License-Identifier: GPL-2.0-only
>> +/* Copyright (c) 2023 Meta, Inc */
>> +#include <linux/bpf.h>
>> +#include <linux/bpf_mem_alloc.h>
>> +#include <linux/btf.h>
>> +#include <linux/btf_ids.h>
>> +#include <linux/filter.h>
>> +#include <linux/scatterlist.h>
>> +#include <linux/skbuff.h>
>> +#include <crypto/skcipher.h>
>> +
>> +/**
>> + * struct bpf_crypto_skcipher_ctx - refcounted BPF sync skcipher 
>> context structure
>> + * @tfm:    The pointer to crypto_sync_skcipher struct.
>> + * @rcu:    The RCU head used to free the crypto context with RCU 
>> safety.
>> + * @usage:    Object reference counter. When the refcount goes to 0, the
>> + *        memory is released back to the BPF allocator, which provides
>> + *        RCU safety.
>> + */
>> +struct bpf_crypto_skcipher_ctx {
>> +    struct crypto_sync_skcipher *tfm;
>> +    struct rcu_head rcu;
>> +    refcount_t usage;
>> +};
>> +
>> +__diag_push();
>> +__diag_ignore_all("-Wmissing-prototypes",
>> +          "Global kfuncs as their definitions will be in BTF");
>> +
>> +static void *__bpf_dynptr_data_ptr(const struct bpf_dynptr_kern *ptr)
>> +{
>> +    enum bpf_dynptr_type type;
>> +
>> +    if (!ptr->data)
>> +        return NULL;
>> +
>> +    type = bpf_dynptr_get_type(ptr);
>> +
>> +    switch (type) {
>> +    case BPF_DYNPTR_TYPE_LOCAL:
>> +    case BPF_DYNPTR_TYPE_RINGBUF:
>> +        return ptr->data + ptr->offset;
>> +    case BPF_DYNPTR_TYPE_SKB:
>> +        return skb_pointer_if_linear(ptr->data, ptr->offset, 
>> __bpf_dynptr_size(ptr));
>> +    case BPF_DYNPTR_TYPE_XDP:
>> +    {
>> +        void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset, 
>> __bpf_dynptr_size(ptr));
> 
> I suspect what it is doing here (for skb and xdp in particular) is very 
> similar to bpf_dynptr_slice. Please check if bpf_dynptr_slice(ptr, 0, 
> NULL, sz) will work.
> 

Well, yes, it's simplified version of bpf_dynptr_slice. The problem is
that bpf_dynptr_slice bpf_kfunc which cannot be used in another
bpf_kfunc. Should I refactor the code to use it in both places? Like
create __bpf_dynptr_slice() which will be internal part of bpf_kfunc?

> 
>> +        if (!IS_ERR_OR_NULL(xdp_ptr))
>> +            return xdp_ptr;
>> +
>> +        return NULL;
>> +    }
>> +    default:
>> +        WARN_ONCE(true, "unknown dynptr type %d\n", type);
>> +        return NULL;
>> +    }
>> +}
>> +
>> +/**
>> + * bpf_crypto_skcipher_ctx_create() - Create a mutable BPF crypto 
>> context.
>> + *
>> + * Allocates a crypto context that can be used, acquired, and 
>> released by
>> + * a BPF program. The crypto context returned by this function must 
>> either
>> + * be embedded in a map as a kptr, or freed with 
>> bpf_crypto_skcipher_ctx_release().
>> + *
>> + * bpf_crypto_skcipher_ctx_create() allocates memory using the BPF 
>> memory
>> + * allocator, and will not block. It may return NULL if no memory is 
>> available.
>> + * @algo: bpf_dynptr which holds string representation of algorithm.
>> + * @key:  bpf_dynptr which holds cipher key to do crypto.
>> + */
>> +__bpf_kfunc struct bpf_crypto_skcipher_ctx *
>> +bpf_crypto_skcipher_ctx_create(const struct bpf_dynptr_kern *palgo,
> 
> Song's patch on __const_str should help on the palgo (which is a string) 
> also:
> https://lore.kernel.org/bpf/20231024235551.2769174-4-song@kernel.org/
> 

Got it, I'll update it.

>> +                   const struct bpf_dynptr_kern *pkey, int *err)
>> +{
>> +    struct bpf_crypto_skcipher_ctx *ctx;
>> +    char *algo;
>> +
>> +    if (__bpf_dynptr_size(palgo) > CRYPTO_MAX_ALG_NAME) {
>> +        *err = -EINVAL;
>> +        return NULL;
>> +    }
>> +
>> +    algo = __bpf_dynptr_data_ptr(palgo);
>> +
>> +    if (!crypto_has_skcipher(algo, CRYPTO_ALG_TYPE_SKCIPHER, 
>> CRYPTO_ALG_TYPE_MASK)) {
>> +        *err = -EOPNOTSUPP;
>> +        return NULL;
>> +    }
>> +
>> +    ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
>> +    if (!ctx) {
>> +        *err = -ENOMEM;
>> +        return NULL;
>> +    }
>> +
>> +    memset(ctx, 0, sizeof(*ctx));
> 
> nit. kzalloc()
> 
>> +
>> +    ctx->tfm = crypto_alloc_sync_skcipher(algo, 0, 0);
>> +    if (IS_ERR(ctx->tfm)) {
>> +        *err = PTR_ERR(ctx->tfm);
>> +        ctx->tfm = NULL;
>> +        goto err;
>> +    }
>> +
>> +    *err = crypto_sync_skcipher_setkey(ctx->tfm, 
>> __bpf_dynptr_data_ptr(pkey),
>> +                       __bpf_dynptr_size(pkey));
>> +    if (*err)
>> +        goto err;
>> +
>> +    refcount_set(&ctx->usage, 1);
>> +
>> +    return ctx;
>> +err:
>> +    if (ctx->tfm)
>> +        crypto_free_sync_skcipher(ctx->tfm);
>> +    kfree(ctx);
>> +
>> +    return NULL;
>> +}
> 
> [ ... ]
> 
>> +static int bpf_crypto_skcipher_crypt(struct crypto_sync_skcipher *tfm,
>> +                     const struct bpf_dynptr_kern *src,
>> +                     struct bpf_dynptr_kern *dst,
>> +                     const struct bpf_dynptr_kern *iv,
>> +                     bool decrypt)
>> +{
>> +    struct skcipher_request *req = NULL;
>> +    struct scatterlist sgin, sgout;
>> +    int err;
>> +
>> +    if (crypto_sync_skcipher_get_flags(tfm) & CRYPTO_TFM_NEED_KEY)
>> +        return -EINVAL;
>> +
>> +    if (__bpf_dynptr_is_rdonly(dst))
>> +        return -EINVAL;
>> +
>> +    if (!__bpf_dynptr_size(dst) || !__bpf_dynptr_size(src))
>> +        return -EINVAL;
>> +
>> +    if (__bpf_dynptr_size(iv) != crypto_sync_skcipher_ivsize(tfm))
>> +        return -EINVAL;
>> +
>> +    req = skcipher_request_alloc(&tfm->base, GFP_ATOMIC);
> 
> Doing alloc per packet may kill performance. Is it possible to optimize 
> it somehow? What is the usual size of the req (e.g. the example in the 
> selftest)?
> 

In ktls code aead_request is allocated every time encryption is invoked, 
see tls_decrypt_sg(), apparently per skb. Doesn't look like performance
killer. For selftest it's only sizeof(struct skcipher_request).

>> +    if (!req)
>> +        return -ENOMEM;
>> +
>> +    sg_init_one(&sgin, __bpf_dynptr_data_ptr(src), 
>> __bpf_dynptr_size(src));
>> +    sg_init_one(&sgout, __bpf_dynptr_data_ptr(dst), 
>> __bpf_dynptr_size(dst));
>> +
>> +    skcipher_request_set_crypt(req, &sgin, &sgout, 
>> __bpf_dynptr_size(src),
>> +                   __bpf_dynptr_data_ptr(iv));
>> +
>> +    err = decrypt ? crypto_skcipher_decrypt(req) : 
>> crypto_skcipher_encrypt(req);
> 
> I am hitting this with the selftest in patch 2:
> 
> [   39.806675] =============================
> [   39.807243] WARNING: suspicious RCU usage
> [   39.807825] 6.6.0-rc7-02091-g17e023652cc1 #336 Tainted: G           O
> [   39.808798] -----------------------------
> [   39.809368] kernel/sched/core.c:10149 Illegal context switch in 
> RCU-bh read-side critical section!
> [   39.810589]
> [   39.810589] other info that might help us debug this:
> [   39.810589]
> [   39.811696]
> [   39.811696] rcu_scheduler_active = 2, debug_locks = 1
> [   39.812616] 2 locks held by test_progs/1769:
> [   39.813249]  #0: ffffffff84dce140 (rcu_read_lock){....}-{1:3}, at: 
> ip6_finish_output2+0x625/0x1b70
> [   39.814539]  #1: ffffffff84dce1a0 (rcu_read_lock_bh){....}-{1:3}, at: 
> __dev_queue_xmit+0x1df/0x2c40
> [   39.815813]
> [   39.815813] stack backtrace:
> [   39.816441] CPU: 1 PID: 1769 Comm: test_progs Tainted: G           O 
> 6.6.0-rc7-02091-g17e023652cc1 #336
> [   39.817774] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), 
> BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
> [   39.819312] Call Trace:
> [   39.819655]  <TASK>
> [   39.819967]  dump_stack_lvl+0x130/0x1d0
> [   39.822669]  dump_stack+0x14/0x20
> [   39.823136]  lockdep_rcu_suspicious+0x220/0x350
> [   39.823777]  __might_resched+0xe0/0x660
> [   39.827915]  __might_sleep+0x89/0xf0
> [   39.828423]  skcipher_walk_virt+0x55/0x120
> [   39.828990]  crypto_ecb_decrypt+0x159/0x310
> [   39.833846]  crypto_skcipher_decrypt+0xa0/0xd0
> [   39.834481]  bpf_crypto_skcipher_crypt+0x29a/0x3c0
> [   39.837100]  bpf_crypto_skcipher_decrypt+0x56/0x70
> [   39.837770]  bpf_prog_fa576505ab32d186_decrypt_sanity+0x180/0x185
> [   39.838627]  cls_bpf_classify+0x3b6/0xf80
> [   39.839807]  tcf_classify+0x2f4/0x550
> 

That's odd. skcipher_walk_virt() checks for SKCIPHER_WALK_SLEEP which
depends on CRYPTO_TFM_REQ_MAY_SLEEP of tfm, which shouldn't be set for
crypto_alloc_sync_skcipher(). I think we need some crypto@ folks to jump
in and explain.

David, Herbert could you please take a look at the patchset to confirm
that crypto API is used properly.

>> +
>> +    skcipher_request_free(req);
>> +
>> +    return err;
>> +}
>> +
> 
> 

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