[PATCH] ARM: select HAVE_POSIX_CPU_TIMERS_TASK_WORK

[Karl Mehltretter <kmehltretter@gmail.com>]

Commit c6e61c06d606 ("ARM: 9463/1: Allow to enable RT") enabled PREEMPT_RT
on ARM but did not select HAVE_POSIX_CPU_TIMERS_TASK_WORK. This leaves
CONFIG_POSIX_CPU_TIMERS_TASK_WORK disabled, so CPU timers expire in hard
IRQ context.

On PREEMPT_RT this makes run_posix_cpu_timers() take the sleeping
sighand->siglock:

  BUG: sleeping function called from invalid context at spinlock_rt.c:48
    rt_spin_lock from lock_task_sighand
    lock_task_sighand from run_posix_cpu_timers
    run_posix_cpu_timers from update_process_times

ARM handles TIF_NOTIFY_RESUME on all return-to-user paths, including v7-M.
ARM32 KVM host support was removed by commit 541ad0150ca4 ("arm: Remove
32bit KVM host support"), so the select need not be conditional on KVM.

Select it to defer POSIX CPU timer expiry to task context.

Reproduced with setrlimit(RLIMIT_CPU, ...) and a busy loop. The same path
is used by setitimer(ITIMER_PROF or ITIMER_VIRTUAL) and POSIX CPU timers
created with timer_create().

Fixes: c6e61c06d606 ("ARM: 9463/1: Allow to enable RT")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Found with a self-hosted syzkaller instance patched to fuzz kernels on
QEMU's versatilepb machine (ARM926EJ-S, ARMv5TE). Runtime-tested before
and after the change on that setup, and compile-tested mps2_defconfig
with PREEMPT_RT (NOMMU, CPU_V7M).

 arch/arm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 9187240a02db..f073b5102d4e 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -133,6 +133,7 @@ config ARM
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
 	select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RSEQ
-- 
2.53.0