netdev - Re: [PATCH net 1/5] net: tls: handle -EBUSY on async encrypt/decrypt requests

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Date: Fri, 8 Sep 2023 17:55:59 +0200
From: Sabrina Dubroca <sd@...asysnail.net>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: netdev@...r.kernel.org, davejwatson@...com, kuba@...nel.org,
	vakul.garg@....com, borisp@...dia.com, john.fastabend@...il.com
Subject: Re: [PATCH net 1/5] net: tls: handle -EBUSY on async encrypt/decrypt
 requests

Thanks for looking at this patch. In retrospect I should have cc'd you
on it.

2023-09-08, 14:10:05 +0800, Herbert Xu wrote:
> Sabrina Dubroca <sd@...asysnail.net> wrote:
> > Since we're setting the CRYPTO_TFM_REQ_MAY_BACKLOG flag on our
> > requests to the crypto API, crypto_aead_{encrypt,decrypt} can return
> > -EBUSY instead of -EINPROGRESS in valid situations. For example, when
> > the cryptd queue for AESNI is full (easy to trigger with an
> > artifically low cryptd.cryptd_max_cpu_qlen), requests will be enqueued
> > to the backlog but still processed. In that case, the async callback
> > will also be called twice: first with err == -EINPROGRESS, which it
> > seems we can just ignore, then with err == 0.
> > 
> > I've only tested this on AESNI with cryptd.
> > 
> > Fixes: a54667f6728c ("tls: Add support for encryption using async offload accelerator")
> > Fixes: 94524d8fc965 ("net/tls: Add support for async decryption of tls records")
> > Signed-off-by: Sabrina Dubroca <sd@...asysnail.net>
> > ---
> > net/tls/tls_sw.c | 23 +++++++++++++++--------
> > 1 file changed, 15 insertions(+), 8 deletions(-)
> 
> You should only use MAY_BACKLOG if you can actually back off and
> stop issuing new requests.  In that case you can only restart
> issuing new requests when the EINPROGRESS notification comes in.
> 
> If that's not the case here you should drop MAY_BACKLOG altogether.

Uh, ok, I didn't know that, thanks for explaining. When I was fixing
this code I couldn't find a mention of what the expectations for
MAY_BACKLOG are. Could you add a comment describing this in the
headers (either for #define CRYPTO_TFM_REQ_MAY_BACKLOG or
aead_request_set_callback, wherever is more appropriate). MAY_BACKLOG
is used by both tls and tipc (talking only about networking) and
neither seem to respect this need to back off.

Jakub, I guess we should drop the CRYPTO_TFM_REQ_MAY_BACKLOG for net,
and maybe consider adding it back (with the back off) in
net-next. Probably not urgent considering that nobody seems to have
run into this bug so far.

But then we have to handle ENOSPC a bit more gracefully, because right
now it looks like
 - on TX, we break the socket (tls_err_abort when tls_do_encryption returns
   an error)
 - on RX, we also break the socket, and we don't decrement
   decrypt_pending so the recv() call gets stuck

Not sure how complex the changes would be, the sendmsg and recvmsg
code is already a bit hard to follow.

-- 
Sabrina