Failed reset of the authentication attempt counter can carry failed attempts over to later sessions

[myxmaster]

Describe your enhancement idea

After a successful login, Lockscreen resets authenticationAttempts to 0 through updateSettings. If that write fails (e.g. the keychain rejects setInternetCredentials), the unlock continues (since #4423 the error is logged), but the persisted counter keeps its value.

The next failed attempt counts up from that stale value (onAttemptLogIn reads the persisted count). Failed attempts from separate sessions therefore add up, and authenticationFailure() wipes the wallet after fewer than maxAuthenticationAttempts (5) failed attempts in one session.

This predates #4423: the reset used to be fire-and-forget, with the same outcome when the write failed.

A possible direction: retry the reset later, e.g. on the next start after a successful login.

[kaloudis]

Hairy one. Don't want to create a security regression.

Possible direction note: should already be implemented. Successfully login should restart any counters.

[myxmaster]

Right, a successful login already resets the counter. This issue only covers the case where that reset write itself fails. The next successful login resets it again, so a stale count only carries over into the next session.

Worst case: 4 failed attempts, then a successful login whose reset write fails, so a single typo in the next session triggers the wipe. That needs a keychain failure on exactly that write, so it's very unlikely.

Maybe a security-neutral option: when the awaited reset fails, retry it a few times in the background (not awaited, so unlocking isn't delayed). It still only writes 0 after a login verified in this session, so nothing is counted more leniently.

Otherwise I'm fine with closing this as won't fix, or keeping it as low priority.