[PATCH net v2] tls: Fix race condition in tls_sw_cancel_work_tx()

[Hyunwoo Kim <imv4bel@gmail.com>]

This issue was discovered during a code audit.

After cancel_delayed_work_sync() is called from tls_sk_proto_close(), 
tx_work_handler() can still be scheduled from paths such as the 
Delayed ACK handler or ksoftirqd.
As a result, the tx_work_handler() worker may dereference a freed 
TLS object.

The following is a simple race scenario:

          cpu0                         cpu1

tls_sk_proto_close()
  tls_sw_cancel_work_tx()
                                 tls_write_space()
                                   tls_sw_write_space()
                                     if (!test_and_set_bit(BIT_TX_SCHEDULED, &tx_ctx->tx_bitmask))
    set_bit(BIT_TX_SCHEDULED, &ctx->tx_bitmask);
    cancel_delayed_work_sync(&ctx->tx_work.work);
                                     schedule_delayed_work(&tx_ctx->tx_work.work, 0);

To prevent this race condition, cancel_delayed_work_sync() is 
replaced with disable_delayed_work_sync().

Fixes: f87e62d45e51 ("net/tls: remove close callback sock unlock/lock around TX work flush")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
Changes in v2:
- Shorten the patch subject
- Target the net tree
- Add the bug discovery background and the race scenario to the commit message
- v1: https://lore.kernel.org/all/aZLotq3aZY0b-dI8@v4bel/
---
 net/tls/tls_sw.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index 9937d4c810f2..b1fa62de9dab 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -2533,7 +2533,7 @@ void tls_sw_cancel_work_tx(struct tls_context *tls_ctx)
 
 	set_bit(BIT_TX_CLOSING, &ctx->tx_bitmask);
 	set_bit(BIT_TX_SCHEDULED, &ctx->tx_bitmask);
-	cancel_delayed_work_sync(&ctx->tx_work.work);
+	disable_delayed_work_sync(&ctx->tx_work.work);
 }
 
 void tls_sw_release_resources_tx(struct sock *sk)
-- 
2.43.0