From: Juraj Šarinay juraj@sarinay.com
[ Upstream commit 21f82062d0f241e55dd59eb630e8710862cc90b4 ]
An exchange with a NFC target must complete within NCI_DATA_TIMEOUT. A delay of 700 ms is not sufficient for cryptographic operations on smart cards. CardOS 6.0 may need up to 1.3 seconds to perform 256-bit ECDH or 3072-bit RSA. To prevent brute-force attacks, passports and similar documents introduce even longer delays into access control protocols (BAC/PACE).
The timeout should be higher, but not too much. The expiration allows us to detect that a NFC target has disappeared.
Signed-off-by: Juraj Šarinay juraj@sarinay.com Reviewed-by: Krzysztof Kozlowski krzysztof.kozlowski@linaro.org Link: https://patch.msgid.link/20250902113630.62393-1-juraj@sarinay.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
YES
Why backport - Fixes real-world timeouts: 700 ms is too short for common smartcard crypto (e.g., ECDH-256, RSA-3072) and ePassports (BAC/PACE) which purposely add delay. This leads to spurious -ETIMEDOUT and failed NFC exchanges for users. - Minimal, contained change: single constant bump from 700 to 3000 ms in a public header, no ABI/API change, no architectural changes, no new feature. - Aligns with existing timeout scale: New value remains below other NCI timeouts like `NCI_CMD_TIMEOUT` (5000 ms) and far below RF deactivate (30000 ms), preserving responsiveness expectations.
What the code change affects - Header adjustment raises the constant used by all data-exchange waits and the data-exchange watchdog timer: - include/net/nfc/nci_core.h:55 changes `#define NCI_DATA_TIMEOUT` to `3000`. - Context shows other timeouts for comparison: `NCI_CMD_TIMEOUT` 5000 ms, `NCI_RF_DEACTIVATE_TIMEOUT` 30000 ms (include/net/nfc/nci_core.h:48-55).
- Data exchange timer: - TX path starts/reset timer with the new value: `mod_timer(&ndev->data_timer, jiffies + msecs_to_jiffies(NCI_DATA_TIMEOUT))` (net/nfc/nci/core.c:1525-1526). - On expiry, it flags a timeout and schedules RX work: `set_bit(NCI_DATA_EXCHANGE_TO, &ndev->flags); queue_work(...)` (net/nfc/nci/core.c:622-628). - RX work completes the pending exchange with -ETIMEDOUT if the flag is set: (net/nfc/nci/core.c:1571-1580). - On successful receive, exchange completion stops the timer cleanly: `timer_delete_sync(&ndev->data_timer)` (net/nfc/nci/data.c:44-46) and delivers the data (net/nfc/nci/data.c:48-60, 262-263).
- Request wait timeouts using the same macro (prevents premature completion timeout during data exchanges and HCI data commands): - HCI send command: `nci_request(..., msecs_to_jiffies(NCI_DATA_TIMEOUT))` (net/nfc/nci/hci.c:244-246). - HCI set/get param: (net/nfc/nci/hci.c:589-591, 628-630). - HCI open pipe: (net/nfc/nci/hci.c:514-516). - NFCC loopback: (net/nfc/nci/core.c:465-467). - Request engine waits up to the supplied timeout: `wait_for_completion_interruptible_timeout(...)` (net/nfc/nci/core.c:112-123), returning -ETIMEDOUT only after the new 3s window.
Risk assessment - Regression risk is low: - Only extends waiting window before declaring timeout; does not alter state machines, packet formats, or driver interfaces. - Timer is consistently cancelled on success; the only user-visible effect is fewer false timeouts on slow targets. - Trade-off: Lost-target detection occurs up to ~3s instead of ~0.7s. Given `NCI_CMD_TIMEOUT` is already 5s and crypto operations commonly exceed 700 ms, 3s is a reasonable balance to avoid false negatives while still detecting vanished targets promptly.
Stable backport criteria - Important bug fix: prevents spurious failures during legitimate NFC operations with smartcards/passports. - Small, localized change with minimal risk and no API/ABI change. - Does not introduce new features or architectural shifts.
Conclusion - Backporting this change will materially improve NFC reliability for users interacting with secure documents and smartcards, with negligible downside.
include/net/nfc/nci_core.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/net/nfc/nci_core.h b/include/net/nfc/nci_core.h index e180bdf2f82b0..664d5058e66e0 100644 --- a/include/net/nfc/nci_core.h +++ b/include/net/nfc/nci_core.h @@ -52,7 +52,7 @@ enum nci_state { #define NCI_RF_DISC_SELECT_TIMEOUT 5000 #define NCI_RF_DEACTIVATE_TIMEOUT 30000 #define NCI_CMD_TIMEOUT 5000 -#define NCI_DATA_TIMEOUT 700 +#define NCI_DATA_TIMEOUT 3000
struct nci_dev;