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@xxxxxxxxxxxxxxx # v3.9
> Signed-off-by: Gustavo Luiz Duarte <gustavold@xxxxxxxxxxxxx>
Applied to powerpc fixes, thanks.
https://git.kernel.org/powerpc/c/2464cc4c345699adea52c7aef75707207cb8a2f6
cheers
