tls: Fix context leak on tls_device_down

The commit cited below claims to fix a use-after-free condition after
tls_device_down. Apparently, the description wasn't fully accurate. The
context stayed alive, but ctx->netdev became NULL, and the offload was
torn down without a proper fallback, so a bug was present, but a
different kind of bug.

Due to misunderstanding of the issue, the original patch dropped the
refcount_dec_and_test line for the context to avoid the alleged
premature deallocation. That line has to be restored, because it matches
the refcount_inc_not_zero from the same function, otherwise the contexts
that survived tls_device_down are leaked.

This patch fixes the described issue by restoring refcount_dec_and_test.
After this change, there is no leak anymore, and the fallback to
software kTLS still works.

Fixes: c55dcdd4 ("net/tls: Fix use-after-free after the TLS device goes down and up")
Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20220512091830.678684-1-maximmi@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/tls/tls_device.c

@@ -1347,7 +1347,10 @@ static int tls_device_down(struct net_device *netdev)
 
 		/* Device contexts for RX and TX will be freed in on sk_destruct
 		 * by tls_device_free_ctx. rx_conf and tx_conf stay in TLS_HW.
+		 * Now release the ref taken above.
 		 */
+		if (refcount_dec_and_test(&ctx->refcount))
+			tls_device_free_ctx(ctx);
 	}
 
 	up_write(&device_offload_lock);