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    SubjectRe: [OOPS s390] Unable to handle kernel pointer dereference at virtual kernel address (null)
    On Fri, Apr 15, 2011 at 08:48:40PM -0500, Jonathan Nieder wrote:
    > > I installed linux-image-2.6.38-2-s390x version 2.6.38-3 on my up-to-date Wheezy
    > > system today. It runs in a virtual machine under z/VM 5.4.0 running in an LPAR
    > > on an IBM z/890. It IPLed just fine. After the IPL, the system fell idle for a while.
    > > Then a CRON job kicked off, which caused a page fault, which caused a kernel oops.
    > > Here is the log:

    Ok, I was able to reproduce it and could verify that my patch fixes the bug.
    Thanks for reporting! The patch below will go upstream:

    Subject: [S390] pfault: fix token handling

    From: Heiko Carstens <heiko.carstens@de.ibm.com>

    f6649a7e "[S390] cleanup lowcore access from external interrupts" changed
    handling of external interrupts. Instead of letting the external interrupt
    handlers accessing the per cpu lowcore the entry code of the kernel reads
    already all fields that are necessary and passes them to the handlers.
    The pfault interrupt handler was incorrectly converted. It tries to
    dereference a value which used to be a pointer to a lowcore field. After
    the conversion however it is not anymore the pointer to the field but its
    content. So instead of a dereference only a cast is needed to get the
    task pointer that caused the pfault.

    Fixes a NULL pointer dereference and a subsequent kernel crash:

    Unable to handle kernel pointer dereference at virtual kernel address (null)
    Oops: 0004 [#1] SMP
    Modules linked in: nfsd exportfs nfs lockd fscache nfs_acl auth_rpcgss sunrpc
    loop qeth_l3 qeth vmur ccwgroup ext3 jbd mbcache dm_mod
    dasd_eckd_mod dasd_diag_mod dasd_mod
    CPU: 0 Not tainted 2.6.38-2-s390x #1
    Process cron (pid: 1106, task: 000000001f962f78, ksp: 000000001fa0f9d0)
    Krnl PSW : 0404200180000000 000000000002c03e (pfault_interrupt+0xa2/0x138)
    R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:0 CC:2 PM:0 EA:3
    Krnl GPRS: 0000000000000000 0000000000000001 0000000000000000 0000000000000001
    000000001f962f78 0000000000518968 0000000090000002 000000001ff03280
    0000000000000000 000000000064f000 000000001f962f78 0000000000002603
    0000000006002603 0000000000000000 000000001ff7fe68 000000001ff7fe48
    Krnl Code: 000000000002c036: 5820d010 l %r2,16(%r13)
    000000000002c03a: 1832 lr %r3,%r2
    000000000002c03c: 1a31 ar %r3,%r1
    >000000000002c03e: ba23d010 cs %r2,%r3,16(%r13)
    000000000002c042: a744fffc brc 4,2c03a
    000000000002c046: a7290002 lghi %r2,2
    000000000002c04a: e320d0000024 stg %r2,0(%r13)
    000000000002c050: 07f0 bcr 15,%r0
    Call Trace:
    ([<000000001f962f78>] 0x1f962f78)
    [<000000000001acda>] do_extint+0xf6/0x138
    [<000000000039b6ca>] ext_no_vtime+0x30/0x34
    [<000000007d706e04>] 0x7d706e04
    Last Breaking-Event-Address:
    [<0000000000000000>] 0x0

    For stable maintainers:
    the first kernel which contains this bug is 2.6.37.

    Reported-by: Stephen Powell <zlinuxman@wowway.com>
    Cc: Jonathan Nieder <jrnieder@gmail.com>
    Cc: stable@kernel.org
    Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
    ---

    arch/s390/mm/fault.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
    index 9217e33..4cf85fe 100644
    --- a/arch/s390/mm/fault.c
    +++ b/arch/s390/mm/fault.c
    @@ -558,9 +558,9 @@ static void pfault_interrupt(unsigned int ext_int_code,
    * Get the token (= address of the task structure of the affected task).
    */
    #ifdef CONFIG_64BIT
    - tsk = *(struct task_struct **) param64;
    + tsk = (struct task_struct *) param64;
    #else
    - tsk = *(struct task_struct **) param32;
    + tsk = (struct task_struct *) param32;
    #endif

    if (subcode & 0x0080) {

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