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    Subject[PATCH 5.4 045/135] powerpc/tm: Fix clearing MSR[TS] in current when reclaiming on signal delivery
    Date
    From: Gustavo Luiz Duarte <gustavold@linux.ibm.com>

    commit 2464cc4c345699adea52c7aef75707207cb8a2f6 upstream.

    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: 2b0a576d15e0 ("powerpc: Add new transactional memory state to the signal context")
    Cc: stable@vger.kernel.org # v3.9
    Signed-off-by: Gustavo Luiz Duarte <gustavold@linux.ibm.com>
    Acked-by: Michael Neuling <mikey@neuling.org>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20200211033831.11165-1-gustavold@linux.ibm.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/powerpc/kernel/signal.c | 17 +++++++++++++++--
    arch/powerpc/kernel/signal_32.c | 28 ++++++++++++++--------------
    arch/powerpc/kernel/signal_64.c | 22 ++++++++++------------
    3 files changed, 39 insertions(+), 28 deletions(-)

    --- a/arch/powerpc/kernel/signal.c
    +++ b/arch/powerpc/kernel/signal.c
    @@ -200,14 +200,27 @@ unsigned long get_tm_stackpointer(struct
    * normal/non-checkpointed stack pointer.
    */

    + unsigned long ret = tsk->thread.regs->gpr[1];
    +
    #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
    BUG_ON(tsk != current);

    if (MSR_TM_ACTIVE(tsk->thread.regs->msr)) {
    + preempt_disable();
    tm_reclaim_current(TM_CAUSE_SIGNAL);
    if (MSR_TM_TRANSACTIONAL(tsk->thread.regs->msr))
    - return tsk->thread.ckpt_regs.gpr[1];
    + ret = tsk->thread.ckpt_regs.gpr[1];
    +
    + /*
    + * If we treclaim, we must clear the current thread's TM bits
    + * before re-enabling preemption. Otherwise we might be
    + * preempted and have the live MSR[TS] changed behind our back
    + * (tm_recheckpoint_new_task() would recheckpoint). Besides, we
    + * enter the signal handler in non-transactional state.
    + */
    + tsk->thread.regs->msr &= ~MSR_TS_MASK;
    + preempt_enable();
    }
    #endif
    - return tsk->thread.regs->gpr[1];
    + return ret;
    }
    --- a/arch/powerpc/kernel/signal_32.c
    +++ b/arch/powerpc/kernel/signal_32.c
    @@ -489,19 +489,11 @@ static int save_user_regs(struct pt_regs
    */
    static int save_tm_user_regs(struct pt_regs *regs,
    struct mcontext __user *frame,
    - struct mcontext __user *tm_frame, int sigret)
    + struct mcontext __user *tm_frame, int sigret,
    + unsigned long msr)
    {
    - unsigned long msr = regs->msr;
    -
    WARN_ON(tm_suspend_disabled);

    - /* Remove TM bits from thread's MSR. The MSR in the sigcontext
    - * just indicates to userland that we were doing a transaction, but we
    - * don't want to return in transactional state. This also ensures
    - * that flush_fp_to_thread won't set TIF_RESTORE_TM again.
    - */
    - regs->msr &= ~MSR_TS_MASK;
    -
    /* Save both sets of general registers */
    if (save_general_regs(&current->thread.ckpt_regs, frame)
    || save_general_regs(regs, tm_frame))
    @@ -912,6 +904,10 @@ int handle_rt_signal32(struct ksignal *k
    int sigret;
    unsigned long tramp;
    struct pt_regs *regs = tsk->thread.regs;
    +#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
    + /* Save the thread's msr before get_tm_stackpointer() changes it */
    + unsigned long msr = regs->msr;
    +#endif

    BUG_ON(tsk != current);

    @@ -944,13 +940,13 @@ int handle_rt_signal32(struct ksignal *k

    #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
    tm_frame = &rt_sf->uc_transact.uc_mcontext;
    - if (MSR_TM_ACTIVE(regs->msr)) {
    + if (MSR_TM_ACTIVE(msr)) {
    if (__put_user((unsigned long)&rt_sf->uc_transact,
    &rt_sf->uc.uc_link) ||
    __put_user((unsigned long)tm_frame,
    &rt_sf->uc_transact.uc_regs))
    goto badframe;
    - if (save_tm_user_regs(regs, frame, tm_frame, sigret))
    + if (save_tm_user_regs(regs, frame, tm_frame, sigret, msr))
    goto badframe;
    }
    else
    @@ -1369,6 +1365,10 @@ int handle_signal32(struct ksignal *ksig
    int sigret;
    unsigned long tramp;
    struct pt_regs *regs = tsk->thread.regs;
    +#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
    + /* Save the thread's msr before get_tm_stackpointer() changes it */
    + unsigned long msr = regs->msr;
    +#endif

    BUG_ON(tsk != current);

    @@ -1402,9 +1402,9 @@ int handle_signal32(struct ksignal *ksig

    #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
    tm_mctx = &frame->mctx_transact;
    - if (MSR_TM_ACTIVE(regs->msr)) {
    + if (MSR_TM_ACTIVE(msr)) {
    if (save_tm_user_regs(regs, &frame->mctx, &frame->mctx_transact,
    - sigret))
    + sigret, msr))
    goto badframe;
    }
    else
    --- a/arch/powerpc/kernel/signal_64.c
    +++ b/arch/powerpc/kernel/signal_64.c
    @@ -192,7 +192,8 @@ static long setup_sigcontext(struct sigc
    static long setup_tm_sigcontexts(struct sigcontext __user *sc,
    struct sigcontext __user *tm_sc,
    struct task_struct *tsk,
    - int signr, sigset_t *set, unsigned long handler)
    + int signr, sigset_t *set, unsigned long handler,
    + unsigned long msr)
    {
    /* When CONFIG_ALTIVEC is set, we _always_ setup v_regs even if the
    * process never used altivec yet (MSR_VEC is zero in pt_regs of
    @@ -207,12 +208,11 @@ static long setup_tm_sigcontexts(struct
    elf_vrreg_t __user *tm_v_regs = sigcontext_vmx_regs(tm_sc);
    #endif
    struct pt_regs *regs = tsk->thread.regs;
    - unsigned long msr = tsk->thread.regs->msr;
    long err = 0;

    BUG_ON(tsk != current);

    - BUG_ON(!MSR_TM_ACTIVE(regs->msr));
    + BUG_ON(!MSR_TM_ACTIVE(msr));

    WARN_ON(tm_suspend_disabled);

    @@ -222,13 +222,6 @@ static long setup_tm_sigcontexts(struct
    */
    msr |= tsk->thread.ckpt_regs.msr & (MSR_FP | MSR_VEC | MSR_VSX);

    - /* Remove TM bits from thread's MSR. The MSR in the sigcontext
    - * just indicates to userland that we were doing a transaction, but we
    - * don't want to return in transactional state. This also ensures
    - * that flush_fp_to_thread won't set TIF_RESTORE_TM again.
    - */
    - regs->msr &= ~MSR_TS_MASK;
    -
    #ifdef CONFIG_ALTIVEC
    err |= __put_user(v_regs, &sc->v_regs);
    err |= __put_user(tm_v_regs, &tm_sc->v_regs);
    @@ -824,6 +817,10 @@ int handle_rt_signal64(struct ksignal *k
    unsigned long newsp = 0;
    long err = 0;
    struct pt_regs *regs = tsk->thread.regs;
    +#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
    + /* Save the thread's msr before get_tm_stackpointer() changes it */
    + unsigned long msr = regs->msr;
    +#endif

    BUG_ON(tsk != current);

    @@ -841,7 +838,7 @@ int handle_rt_signal64(struct ksignal *k
    err |= __put_user(0, &frame->uc.uc_flags);
    err |= __save_altstack(&frame->uc.uc_stack, regs->gpr[1]);
    #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
    - if (MSR_TM_ACTIVE(regs->msr)) {
    + if (MSR_TM_ACTIVE(msr)) {
    /* The ucontext_t passed to userland points to the second
    * ucontext_t (for transactional state) with its uc_link ptr.
    */
    @@ -849,7 +846,8 @@ int handle_rt_signal64(struct ksignal *k
    err |= setup_tm_sigcontexts(&frame->uc.uc_mcontext,
    &frame->uc_transact.uc_mcontext,
    tsk, ksig->sig, NULL,
    - (unsigned long)ksig->ka.sa.sa_handler);
    + (unsigned long)ksig->ka.sa.sa_handler,
    + msr);
    } else
    #endif
    {

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