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    Subject[PATCH 3.16 005/136] s390/runtime instrumention: fix possible memory corruption
    3.16.54-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

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

    commit d6e646ad7cfa7034d280459b2b2546288f247144 upstream.

    For PREEMPT enabled kernels the runtime instrumentation (RI) code
    contains a possible use-after-free bug. If a task that makes use of RI
    exits, it will execute do_exit() while still enabled for preemption.

    That function will call exit_thread_runtime_instr() via
    exit_thread(). If exit_thread_runtime_instr() gets preempted after the
    RI control block of the task has been freed but before the pointer to
    it is set to NULL, then save_ri_cb(), called from switch_to(), will
    write to already freed memory.

    Avoid this and simply disable preemption while freeing the control
    block and setting the pointer to NULL.

    Fixes: e4b8b3f33fca ("s390: add support for runtime instrumentation")
    Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
    Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
    Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
    [bwh: Backported to 3.16: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/s390/kernel/runtime_instr.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/arch/s390/kernel/runtime_instr.c
    +++ b/arch/s390/kernel/runtime_instr.c
    @@ -53,12 +53,14 @@ void exit_thread_runtime_instr(void)
    {
    struct task_struct *task = current;

    + preempt_disable();
    if (!task->thread.ri_cb)
    return;
    disable_runtime_instr();
    kfree(task->thread.ri_cb);
    task->thread.ri_signum = 0;
    task->thread.ri_cb = NULL;
    + preempt_enable();
    }

    static void runtime_instr_int_handler(struct ext_code ext_code,
    @@ -100,9 +102,7 @@ SYSCALL_DEFINE2(s390_runtime_instr, int,
    return -EOPNOTSUPP;

    if (command == S390_RUNTIME_INSTR_STOP) {
    - preempt_disable();
    exit_thread_runtime_instr();
    - preempt_enable();
    return 0;
    }

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