• Gustavo Luiz Duarte's avatar
    powerpc/tm: Fix clearing MSR[TS] in current when reclaiming on signal delivery · 2464cc4c
    Gustavo Luiz Duarte authored
    After a treclaim, we expect to be in non-transactional state. If we
    don't clear the current thread's MSR[TS] before we get preempted, then
    tm_recheckpoint_new_task() will recheckpoint and we get rescheduled in
    suspended transaction state.
    
    When handling a signal caught in transactional state,
    handle_rt_signal64() calls get_tm_stackpointer() that treclaims the
    transaction using tm_reclaim_current() but without clearing the
    thread's MSR[TS]. This can cause the TM Bad Thing exception below if
    later we pagefault and get preempted trying to access the user's
    sigframe, using __put_user(). Afterwards, when we are rescheduled back
    into do_page_fault() (but now in suspended state since the thread's
    MSR[TS] was not cleared), upon executing 'rfid' after completion of
    the page fault handling, the exception is raised because a transition
    from suspended to non-transactional state is invalid.
    
      Unexpected TM Bad Thing exception at c00000000000de44 (msr 0x8000000302a03031) tm_scratch=800000010280b033
      Oops: Unrecoverable exception, sig: 6 [#1]
      LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
      CPU: 25 PID: 15547 Comm: a.out Not tainted 5.4.0-rc2 #32
      NIP:  c00000000000de44 LR: c000000000034728 CTR: 0000000000000000
      REGS: c00000003fe7bd70 TRAP: 0700   Not tainted  (5.4.0-rc2)
      MSR:  8000000302a03031 <SF,VEC,VSX,FP,ME,IR,DR,LE,TM[SE]>  CR: 44000884  XER: 00000000
      CFAR: c00000000000dda4 IRQMASK: 0
      PACATMSCRATCH: 800000010280b033
      GPR00: c000000000034728 c000000f65a17c80 c000000001662800 00007fffacf3fd78
      GPR04: 0000000000001000 0000000000001000 0000000000000000 c000000f611f8af0
      GPR08: 0000000000000000 0000000078006001 0000000000000000 000c000000000000
      GPR12: c000000f611f84b0 c00000003ffcb200 0000000000000000 0000000000000000
      GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
      GPR20: 0000000000000000 0000000000000000 0000000000000000 c000000f611f8140
      GPR24: 0000000000000000 00007fffacf3fd68 c000000f65a17d90 c000000f611f7800
      GPR28: c000000f65a17e90 c000000f65a17e90 c000000001685e18 00007fffacf3f000
      NIP [c00000000000de44] fast_exception_return+0xf4/0x1b0
      LR [c000000000034728] handle_rt_signal64+0x78/0xc50
      Call Trace:
      [c000000f65a17c80] [c000000000034710] handle_rt_signal64+0x60/0xc50 (unreliable)
      [c000000f65a17d30] [c000000000023640] do_notify_resume+0x330/0x460
      [c000000f65a17e20] [c00000000000dcc4] ret_from_except_lite+0x70/0x74
      Instruction dump:
      7c4ff120 e8410170 7c5a03a6 38400000 f8410060 e8010070 e8410080 e8610088
      60000000 60000000 e8810090 e8210078 <4c000024> 48000000 e8610178 88ed0989
      ---[ end trace 93094aa44b442f87 ]---
    
    The simplified sequence of events that triggers the above exception is:
    
      ...				# userspace in NON-TRANSACTIONAL state
      tbegin			# userspace in TRANSACTIONAL state
      signal delivery		# kernelspace in SUSPENDED state
      handle_rt_signal64()
        get_tm_stackpointer()
          treclaim			# kernelspace in NON-TRANSACTIONAL state
        __put_user()
          page fault happens. We will never get back here because of the TM Bad Thing exception.
    
      page fault handling kicks in and we voluntarily preempt ourselves
      do_page_fault()
        __schedule()
          __switch_to(other_task)
    
      our task is rescheduled and we recheckpoint because the thread's MSR[TS] was not cleared
      __switch_to(our_task)
        switch_to_tm()
          tm_recheckpoint_new_task()
            trechkpt			# kernelspace in SUSPENDED state
    
      The page fault handling resumes, but now we are in suspended transaction state
      do_page_fault()    completes
      rfid     <----- trying to get back where the page fault happened (we were non-transactional back then)
      TM Bad Thing			# illegal transition from suspended to non-transactional
    
    This patch fixes that issue by clearing the current thread's MSR[TS]
    just after treclaim in get_tm_stackpointer() so that we stay in
    non-transactional state in case we are preempted. In order to make
    treclaim and clearing the thread's MSR[TS] atomic from a preemption
    perspective when CONFIG_PREEMPT is set, preempt_disable/enable() is
    used. It's also necessary to save the previous value of the thread's
    MSR before get_tm_stackpointer() is called so that it can be exposed
    to the signal handler later in setup_tm_sigcontexts() to inform the
    userspace MSR at the moment of the signal delivery.
    
    Found with tm-signal-context-force-tm kernel selftest.
    
    Fixes: 2b0a576d ("powerpc: Add new transactional memory state to the signal context")
    Cc: stable@vger.kernel.org # v3.9
    Signed-off-by: default avatarGustavo Luiz Duarte <gustavold@linux.ibm.com>
    Acked-by: default avatarMichael Neuling <mikey@neuling.org>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20200211033831.11165-1-gustavold@linux.ibm.com
    2464cc4c
signal_64.c 28.2 KB