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From:   David Howells <dhowells@redhat.com>
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References: <ZJEwPLudZlrInzYs@gondor.apana.org.au> <427646.1686913832@warthog.procyon.org.uk>
To:     Herbert Xu <herbert@gondor.apana.org.au>
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        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
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Subject: Re: [PATCH net-next v2] crypto: af_alg/hash: Fix recvmsg() after sendmsg(MSG_MORE)
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Date:   Tue, 20 Jun 2023 08:42:15 +0100
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Herbert Xu <herbert@gondor.apana.org.au> wrote:

> > +		hash_free_result(sk, ctx);
> 
> Please revert this change as I explained in the other message.
> 
> > +		if (!msg_data_left(msg))
> > +			goto done; /* Zero-length; don't start new req */
> 
> This is still broken in the case of a zero-length message with
> MSG_MORE set.  Here you will short-circuit out without ever calling
> crypto_ahash_init.  However, hash_recvmsg will directly call
> crypto_ahash_final on this, which is undefined.

Not so.  hash_recvmsg() will call crypto_ahash_init() first because ctx->more
is false (hence why we came down this branch in hash_sendmsg()) and the result
was released on the previous line (which you're objecting to).  If it goes to
the "done" label, it will skip setting ctx->more to true if MSG_MORE is
passed.

However, given you want sendmsg() to do the init->digest cycle on zero length
data, I think we should revert to the previous version of the patch that makes
a pass of the loop even with no data.

David