• Rob Gardner's avatar
    sparc64: Prevent perf from running during super critical sections · 6309eb77
    Rob Gardner authored
    commit fc290a11 upstream.
    
    This fixes another cause of random segfaults and bus errors that may
    occur while running perf with the callgraph option.
    
    Critical sections beginning with spin_lock_irqsave() raise the interrupt
    level to PIL_NORMAL_MAX (14) and intentionally do not block performance
    counter interrupts, which arrive at PIL_NMI (15).
    
    But some sections of code are "super critical" with respect to perf
    because the perf_callchain_user() path accesses user space and may cause
    TLB activity as well as faults as it unwinds the user stack.
    
    One particular critical section occurs in switch_mm:
    
            spin_lock_irqsave(&mm->context.lock, flags);
            ...
            load_secondary_context(mm);
            tsb_context_switch(mm);
            ...
            spin_unlock_irqrestore(&mm->context.lock, flags);
    
    If a perf interrupt arrives in between load_secondary_context() and
    tsb_context_switch(), then perf_callchain_user() could execute with
    the context ID of one process, but with an active TSB for a different
    process. When the user stack is accessed, it is very likely to
    incur a TLB miss, since the h/w context ID has been changed. The TLB
    will then be reloaded with a translation from the TSB for one process,
    but using a context ID for another process. This exposes memory from
    one process to another, and since it is a mapping for stack memory,
    this usually causes the new process to crash quickly.
    
    This super critical section needs more protection than is provided
    by spin_lock_irqsave() since perf interrupts must not be allowed in.
    
    Since __tsb_context_switch already goes through the trouble of
    disabling interrupts completely, we fix this by moving the secondary
    context load down into this better protected region.
    
    Orabug: 25577560
    Signed-off-by: default avatarDave Aldridge <david.j.aldridge@oracle.com>
    Signed-off-by: default avatarRob Gardner <rob.gardner@oracle.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    6309eb77
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