netdev - Re: [PATCH net-next] net/tls: fix race condition causing kernel panic

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Date:   Wed, 20 May 2020 22:39:11 +0530
From:   Vinay Kumar Yadav <vinay.yadav@...lsio.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, kuba@...nel.org, secdev@...lsio.com
Subject: Re: [PATCH net-next] net/tls: fix race condition causing kernel panic

David

On 5/20/2020 12:46 AM, David Miller wrote:
> From: Vinay Kumar Yadav <vinay.yadav@...lsio.com>
> Date: Tue, 19 May 2020 13:13:27 +0530
>
>> +		spin_lock_bh(&ctx->encrypt_compl_lock);
>> +		pending = atomic_read(&ctx->encrypt_pending);
>> +		spin_unlock_bh(&ctx->encrypt_compl_lock);
> The sequence:
>
> 	lock();
> 	x = p->y;
> 	unlock();
>
> Does not fix anything, and is superfluous locking.
>
> The value of p->y can change right after the unlock() call, so you
> aren't protecting the atomic'ness of the read and test sequence
> because the test is outside of the lock.

Here, by using lock I want to achieve atomicity of following statements.

pending = atomic_dec_return(&ctx->decrypt_pending);
       if (!pending && READ_ONCE(ctx->async_notify))
            complete(&ctx->async_wait.completion);

means, don't want to read (atomic_read(&ctx->decrypt_pending))
in middle of two statements

atomic_dec_return(&ctx->decrypt_pending);
and
complete(&ctx->async_wait.completion);

Why am I protecting only read, not test ?

complete() is called only if pending == 0
if we read atomic_read(&ctx->decrypt_pending) = 0
that means complete() is already called and its okay to
initialize completion (reinit_completion(&ctx->async_wait.completion))

if we read atomic_read(&ctx->decrypt_pending) as non zero that means:
1- complete() is going to be called or
2- complete() already called (if we read atomic_read(&ctx->decrypt_pending) == 1, then complete() is called just after unlock())
for both scenario its okay to go into wait (crypto_wait_req(-EINPROGRESS, &ctx->async_wait))


Thanks,
Vinay