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    Subject[PATCH 4.9 019/152] ptrace: fix PTRACE_LISTEN race corrupting task->state
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

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

    From: bsegall@google.com <bsegall@google.com>

    commit 5402e97af667e35e54177af8f6575518bf251d51 upstream.

    In PT_SEIZED + LISTEN mode STOP/CONT signals cause a wakeup against
    __TASK_TRACED. If this races with the ptrace_unfreeze_traced at the end
    of a PTRACE_LISTEN, this can wake the task /after/ the check against
    __TASK_TRACED, but before the reset of state to TASK_TRACED. This
    causes it to instead clobber TASK_WAKING, allowing a subsequent wakeup
    against TRACED while the task is still on the rq wake_list, corrupting
    it.

    Oleg said:
    "The kernel can crash or this can lead to other hard-to-debug problems.
    In short, "task->state = TASK_TRACED" in ptrace_unfreeze_traced()
    assumes that nobody else can wake it up, but PTRACE_LISTEN breaks the
    contract. Obviusly it is very wrong to manipulate task->state if this
    task is already running, or WAKING, or it sleeps again"

    [akpm@linux-foundation.org: coding-style fixes]
    Fixes: 9899d11f ("ptrace: ensure arch_ptrace/ptrace_request can never race with SIGKILL")
    Link: http://lkml.kernel.org/r/xm26y3vfhmkp.fsf_-_@bsegall-linux.mtv.corp.google.com
    Signed-off-by: Ben Segall <bsegall@google.com>
    Acked-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/ptrace.c | 14 ++++++++++----
    1 file changed, 10 insertions(+), 4 deletions(-)

    --- a/kernel/ptrace.c
    +++ b/kernel/ptrace.c
    @@ -181,11 +181,17 @@ static void ptrace_unfreeze_traced(struc

    WARN_ON(!task->ptrace || task->parent != current);

    + /*
    + * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely.
    + * Recheck state under the lock to close this race.
    + */
    spin_lock_irq(&task->sighand->siglock);
    - if (__fatal_signal_pending(task))
    - wake_up_state(task, __TASK_TRACED);
    - else
    - task->state = TASK_TRACED;
    + if (task->state == __TASK_TRACED) {
    + if (__fatal_signal_pending(task))
    + wake_up_state(task, __TASK_TRACED);
    + else
    + task->state = TASK_TRACED;
    + }
    spin_unlock_irq(&task->sighand->siglock);
    }


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