netdev - Re: [PATCH V2] Fix a sleep-in-atomic bug in shash_setkey

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Date:   Tue, 3 Oct 2017 09:46:04 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Jia-Ju Bai <baijiaju1990@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Neil Horman <nhorman@...driver.com>, vyasevich@...il.com,
        Kalle Valo <kvalo@...eaurora.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        linux-sctp@...r.kernel.org,
        Linux Wireless List <linux-wireless@...r.kernel.org>
Subject: Re: [PATCH V2] Fix a sleep-in-atomic bug in shash_setkey_unaligned

On Mon, Oct 2, 2017 at 10:26 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Mon, Oct 02, 2017 at 09:18:24PM -0700, Andy Lutomirski wrote:
>> > On Oct 2, 2017, at 7:25 PM, Jia-Ju Bai <baijiaju1990@....com> wrote:
>> >
>> > The SCTP program may sleep under a spinlock, and the function call path is:
>> > sctp_generate_t3_rtx_event (acquire the spinlock)
>> >  sctp_do_sm
>> >    sctp_side_effects
>> >      sctp_cmd_interpreter
>> >        sctp_make_init_ack
>> >          sctp_pack_cookie
>> >            crypto_shash_setkey
>> >              shash_setkey_unaligned
>> >                kmalloc(GFP_KERNEL)
>>
>> I'm going to go out on a limb here: why on Earth is out crypto API so
>> full of indirection that we allocate memory at all here?
>
> The crypto API operates on a one key per-tfm basis.  So normally
> tfm allocation and key setting is done once only and not done on
> the data path.
>
> I have looked at the SCTP code and it appears to fit this paradigm.
> That is, we should be able to allocate the tfm and set the key when
> the key is actually generated via get_random_bytes, rather than every
> time the key is used which is not only a waste but as you see runs
> into API issues.

It's a waste because it loses a pre-computation advantage.

The fact that it has memory allocation issues is crypto API's fault,
full stop.  There is no legit reason to need to allocate anything.