Re: [PATCH v3 1/3] powerpc/tm: Fix clearing MSR[TS] in current when reclaiming on signal delivery

On Tue, 2020-02-11 at 03:38:29 UTC, Gustavo Luiz Duarte wrote:

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
	Modules linked in: nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip6_tables ip_tables nft_compat ip_set nf_tables nfnetlink xts vmx_crypto sg virtio_balloon
	r_mod cdrom virtio_net net_failover virtio_blk virtio_scsi failover dm_mirror dm_region_hash dm_log dm_mod
	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.

v3: Subject and comment improvements.
v2: Fix build failure when tm is disabled.

Fixes: 2b0a576d15e0 ("powerpc: Add new transactional memory state to the signal context")
Cc: stable@vger.kernel.org # v3.9
Signed-off-by: Gustavo Luiz Duarte <redacted>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/2464cc4c345699adea52c7aef75707207cb8a2f6

cheers