netdev - Re: [PATCH] net/tls: Fix slab-use-after-free in tls_encrypt

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Date: Tue, 17 Oct 2023 18:25:12 +0800
From: Juntong Deng <juntong.deng@...look.com>
To: Simon Horman <horms@...nel.org>
Cc: borisp@...dia.com, john.fastabend@...il.com, kuba@...nel.org,
 davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kernel-mentees@...ts.linuxfoundation.org,
 syzbot+29c22ea2d6b2c5fd2eae@...kaller.appspotmail.com
Subject: Re: [PATCH] net/tls: Fix slab-use-after-free in tls_encrypt_done

On 2023/10/16 17:50, Simon Horman wrote:
> On Thu, Oct 12, 2023 at 07:02:51PM +0800, Juntong Deng wrote:
>> In the current implementation, ctx->async_wait.completion is completed
>> after spin_lock_bh, which causes tls_sw_release_resources_tx to
>> continue executing and return to tls_sk_proto_cleanup, then return
> 
> Hi Juntong Deng,
> 
> I'm slightly confused by "causes tls_sw_release_resources_tx to  continue
> executing".
> 
> What I see in tls_sw_release_resources_tx() is:
> 
>          /* Wait for any pending async encryptions to complete */
>          spin_lock_bh(&ctx->encrypt_compl_lock);
>          ctx->async_notify = true;
>          pending = atomic_read(&ctx->encrypt_pending);
>          spin_unlock_bh(&ctx->encrypt_compl_lock);
> 
> Am I wrong in thinking the above will block because
> (the same) ctx->encrypt_compl_lock is held in tls_encrypt_done?
> 

Hi Simon Horman,

What I mean is that tls_sw_release_resources_tx will pause at
crypto_wait_req(-EINPROGRESS, &ctx->async_wait) because crypto_wait_req
call wait_for_completion.

Then after tls_encrypt_done call complete(&ctx->async_wait.completion),
it will cause tls_sw_release_resources_tx to continue executing.

>> to tls_sk_proto_close, and after that enter tls_sw_free_ctx_tx to kfree
>> the entire struct tls_context (including ctx->encrypt_compl_lock).
>>
>> Since ctx->encrypt_compl_lock has been freed, subsequent spin_unlock_bh
>> will result in slab-use-after-free error. Due to SMP, even using
>> spin_lock_bh does not prevent tls_sw_release_resources_tx from continuing
>> on other CPUs. After tls_sw_release_resources_tx is woken up, there is no
>> attempt to hold ctx->encrypt_compl_lock again, therefore everything
>> described above is possible.
>>
>> The fix is to put complete(&ctx->async_wait.completion) after
>> spin_unlock_bh, making the release after the unlock. Since complete is
>> only executed if pending is 0, which means this is the last record, there
>> is no need to worry about race condition causing duplicate completes.
>>
>> Reported-by: syzbot+29c22ea2d6b2c5fd2eae@...kaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=29c22ea2d6b2c5fd2eae
>> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
>> ---
>>   net/tls/tls_sw.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
>> index 270712b8d391..7abe5a6aa989 100644
>> --- a/net/tls/tls_sw.c
>> +++ b/net/tls/tls_sw.c
>> @@ -441,6 +441,7 @@ static void tls_encrypt_done(void *data, int err)
>>   	struct sk_msg *msg_en;
>>   	bool ready = false;
>>   	struct sock *sk;
>> +	int async_notify;
>>   	int pending;
>>   
>>   	msg_en = &rec->msg_encrypted;
>> @@ -482,10 +483,11 @@ static void tls_encrypt_done(void *data, int err)
>>   
>>   	spin_lock_bh(&ctx->encrypt_compl_lock);
>>   	pending = atomic_dec_return(&ctx->encrypt_pending);
>> +	async_notify = ctx->async_notify;
>> +	spin_unlock_bh(&ctx->encrypt_compl_lock);
>>   
>> -	if (!pending && ctx->async_notify)
>> +	if (!pending && async_notify)
>>   		complete(&ctx->async_wait.completion);
>> -	spin_unlock_bh(&ctx->encrypt_compl_lock);
>>   
>>   	if (!ready)
>>   		return;
>> -- 
>> 2.39.2
>>
>>