[PATCH v3] powerpc: signals: Discard transaction state from signal frames

[Laurent Dufour]

On 23/08/2016 02:46, Cyril Bur wrote:
> Userspace can begin and suspend a transaction within the signal
> handler which means they might enter sys_rt_sigreturn() with the
> processor in suspended state.
> 
> sys_rt_sigreturn() wants to restore process context (which may have
> been in a transaction before signal delivery). To do this it must
> restore TM SPRS. To achieve this, any transaction initiated within the
> signal frame must be discarded in order to be able to restore TM SPRs
> as TM SPRs can only be manipulated non-transactionally..
> From the PowerPC ISA:
>   TM Bad Thing Exception [Category: Transactional Memory]
>    An attempt is made to execute a mtspr targeting a TM register in
>    other than Non-transactional state.
> 
> Not doing so results in a TM Bad Thing:
> [12045.221359] Kernel BUG at c000000000050a40 [verbose debug info unavailable]
> [12045.221470] Unexpected TM Bad Thing exception at c000000000050a40 (msr 0x201033)
> [12045.221540] Oops: Unrecoverable exception, sig: 6 [#1]
> [12045.221586] SMP NR_CPUS=2048 NUMA PowerNV
> [12045.221634] Modules linked in: xt_CHECKSUM iptable_mangle ipt_MASQUERADE
>  nf_nat_masquerade_ipv4 iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4
>  xt_conntrack nf_conntrack ipt_REJECT nf_reject_ipv4 xt_tcpudp bridge stp llc ebtable_filter
>  ebtables ip6table_filter ip6_tables iptable_filter ip_tables x_tables kvm_hv kvm
>  uio_pdrv_genirq ipmi_powernv uio powernv_rng ipmi_msghandler autofs4 ses enclosure
>  scsi_transport_sas bnx2x ipr mdio libcrc32c
> [12045.222167] CPU: 68 PID: 6178 Comm: sigreturnpanic Not tainted 4.7.0 #34
> [12045.222224] task: c0000000fce38600 ti: c0000000fceb4000 task.ti: c0000000fceb4000
> [12045.222293] NIP: c000000000050a40 LR: c0000000000163bc CTR: 0000000000000000
> [12045.222361] REGS: c0000000fceb7ac0 TRAP: 0700   Not tainted (4.7.0)
> [12045.222418] MSR: 9000000300201033 <SF,HV,ME,IR,DR,RI,LE,TM[SE]> CR: 28444280  XER: 20000000
> [12045.222625] CFAR: c0000000000163b8 SOFTE: 0 PACATMSCRATCH: 900000014280f033
> GPR00: 01100000b8000001 c0000000fceb7d40 c00000000139c100 c0000000fce390d0
> GPR04: 900000034280f033 0000000000000000 0000000000000000 0000000000000000
> GPR08: 0000000000000000 b000000000001033 0000000000000001 0000000000000000
> GPR12: 0000000000000000 c000000002926400 0000000000000000 0000000000000000
> GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
> GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
> GPR24: 0000000000000000 00003ffff98cadd0 00003ffff98cb470 0000000000000000
> GPR28: 900000034280f033 c0000000fceb7ea0 0000000000000001 c0000000fce390d0
> [12045.223535] NIP [c000000000050a40] tm_restore_sprs+0xc/0x1c
> [12045.223584] LR [c0000000000163bc] tm_recheckpoint+0x5c/0xa0
> [12045.223630] Call Trace:
> [12045.223655] [c0000000fceb7d80] [c000000000026e74] sys_rt_sigreturn+0x494/0x6c0
> [12045.223738] [c0000000fceb7e30] [c0000000000092e0] system_call+0x38/0x108
> [12045.223806] Instruction dump:
> [12045.223841] 7c800164 4e800020 7c0022a6 f80304a8 7c0222a6 f80304b0 7c0122a6 f80304b8
> [12045.223955] 4e800020 e80304a8 7c0023a6 e80304b0 <7c0223a6> e80304b8 7c0123a6 4e800020
> [12045.224074] ---[ end trace cb8002ee240bae76 ]---
> 
> It isn't clear exactly if there is really a use case for userspace
> returning with a suspended transaction, however, doing so doesn't (on
> its own) constitute a bad frame. As such, this patch simply discards
> the transactional state of the context calling the sigreturn and
> continues.
> 
> Reported-by: Laurent Dufour <ldufour at linux.vnet.ibm.com>
> Signed-off-by: Cyril Bur <cyrilbur at gmail.com>
> Tested-by: Laurent Dufour <ldufour at linux.vnet.ibm.com>

Reviewed-by: Laurent Dufour <ldufour at linux.vnet.ibm.com>