tls: adjust recv return with async crypto and failed copy to userspace

process_rx_list may not copy as many bytes as we want to the userspace
buffer, for example in case we hit an EFAULT during the copy. If this
happens, we should only count the bytes that were actually copied,
which may be 0.

Subtracting async_copy_bytes is correct in both peek and !peek cases,
because decrypted == async_copy_bytes + peeked for the peek case: peek
is always !ZC, and we can go through either the sync or async path. In
the async case, we add chunk to both decrypted and
async_copy_bytes. In the sync case, we add chunk to both decrypted and
peeked. I missed that in commit 6caaf10 ("tls: fix peeking with
sync+async decryption").

Fixes: 4d42cd6 ("tls: rx: fix return value for async crypto")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/1b5a1eaab3c088a9dd5d9f1059ceecd7afe888d1.1711120964.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/tls/tls_sw.c

@@ -2158,6 +2158,9 @@ int tls_sw_recvmsg(struct sock *sk,
 		else
 			err = process_rx_list(ctx, msg, &control, 0,
 					      async_copy_bytes, is_peek, NULL);
+
+		/* we could have copied less than we wanted, and possibly nothing */
+		decrypted += max(err, 0) - async_copy_bytes;
 	}
 
 	copied += decrypted;