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VM66301: CP SAVE AREA MISUSE

A fix is available

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APAR status

  • Closed as program error.

Error description

  • z/VM code for managing guest pages on PAGE space uses a save
    area without proper serialization.
    
    The z/VM control program (CP) storage management
    code for managing guest pages on PAGE space uses a
    save area without proper serialization.
    With certain timing conditions, it could be referencing
    a save area that has already been
    released and/or re-used for another purpose.  This can lead
    to a variety of possible symptoms, such as save area queue
    mismanagement, modification of another task's save area,
    incorrect logic decisions by the paging code, or host page
    faults.
    
    Other Keywords: SAVBK SVGBK CPEBK PPIBK
    

Local fix

  • N/A
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED: All users of z/VM                            *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    ****************************************************************
    * RECOMMENDATION: APPLY PTF                                    *
    ****************************************************************
    z/VM Control Program (CP) storage management code for managing
    guest pages on PAGE space (module HCPPAG ASSEMBLE) improperly
    accesses its save area (SAVBK/SVGBK/CPEBK/PPIBK).  It does so
    after turning control of its save area over to I/O code that
    performs the read/write of the guest page.  When the I/O
    completes the interrupt handler will stack the return to
    HCPPAG's caller.
    
    However, in very rare timing windows, it's possible for the
    I/O to complete and return to the caller before HCPPAG
    completes its processing and exits to the dispatcher.  HCPPAG
    continues to use SVGWRK fields in this save area, which might
    have since been re-used.  A further condition required to
    expose this problem is that it must be a system task (SYSTEMMP)
    that invokes the HCPPAG interface to read/write a guest page.
    
    Possible symptoms of this are save area queue mismanagement,
    modification of another task's save area, incorrect logic
    decisions by the paging code, or host page faults.
    

Problem conclusion

  • A new PPIBK field (the PPIBK is a remapping/re-labeling of SVGBK
    and SVGWRK fields), is used by HCPPAG to indicate its continued
    use of the save area.  HCPPAG uses TS prior to invoking paging
    I/O routines, and does not release/clear the field until it
    has completed its use of the save area.
    
    The paging I/O interrupt handler (HCPPAH) uses this field to
    detect when HCPPAG is still actively using the save area and
    delays stacking the save area to return to HCPPAG's caller until
    HCPPAG has released/cleared the field.
    
    This ensures that the save area will not be released and re-used
    until after HCPPAG completes its use of it.
    
    Since this only happens in such small/rare timing windows, most
    of the time HCPPAH will be able to stack the save area
    immediately, with no delay, as HCPPAG will already be done with
    it by the time the I/O completes.
    

Temporary fix

  • *********
    * HIPER *
    *********
    FOR RELEASE VM/ESACP/ESAR710 :
    PREREQ: VM66263
    CO-REQ: NONE
    IF-REQ: NONE
    

Comments

  • ×**** PE19/08/29 FIX IN ERROR. SEE APAR VM66319  FOR DESCRIPTION
    

APAR Information

  • APAR number

    VM66301

  • Reported component name

    VM CP

  • Reported component ID

    568411202

  • Reported release

    710

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    YesHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2019-07-01

  • Closed date

    2019-07-29

  • Last modified date

    2019-09-18

  • APAR is sysrouted FROM one or more of the following:

  • APAR is sysrouted TO one or more of the following:

    UM35496

Modules/Macros

  • HCPPAG   HCPPAH   HCPPPIBK
    

Fix information

  • Fixed component name

    VM CP

  • Fixed component ID

    568411202

Applicable component levels

  • R710 PSY UM35496

       UP19/07/31 I 1000 ¢

Fix is available

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Document information

More support for: z/VM family

Software version: 710

Operating system(s): z/VM

Reference #: VM66301

Modified date: 18 September 2019