posix-timers: Ensure timer ID search-loop limit is valid

In the Linux kernel, the following vulnerability has been resolved:

posix-timers: Ensure timer ID search-loop limit is valid

posix_timer_add() tries to allocate a posix timer ID by starting from the

cached ID which was stored by the last successful allocation.

This is done in a loop searching the ID space for a free slot one by

one. The loop has to terminate when the search wrapped around to the

starting point.

But that's racy vs. establishing the starting point. That is read out

lockless, which leads to the following problem:

CPU0 CPU1

posix_timer_add()

start = sig->posix_timer_id;

lock(hash_lock);

... posix_timer_add()

if (++sig->posix_timer_id < 0)

start = sig->posix_timer_id;

sig->posix_timer_id = 0;

So CPU1 can observe a negative start value, i.e. -1, and the loop break

never happens because the condition can never be true:

if (sig->posix_timer_id == start)

break;

While this is unlikely to ever turn into an endless loop as the ID space is

huge (INT_MAX), the racy read of the start value caught the attention of

KCSAN and Dmitry unearthed that incorrectness.

Rewrite it so that all id operations are under the hash lock.