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    Subject[tip:core/urgent] sys: Fix missing rcu protection for __task_cred() access
    Commit-ID:  d4581a239a40319205762b76c01eb6363f277efa
    Gitweb: http://git.kernel.org/tip/d4581a239a40319205762b76c01eb6363f277efa
    Author: Thomas Gleixner <tglx@linutronix.de>
    AuthorDate: Thu, 10 Dec 2009 00:52:51 +0000
    Committer: Thomas Gleixner <tglx@linutronix.de>
    CommitDate: Thu, 10 Dec 2009 23:04:11 +0100

    sys: Fix missing rcu protection for __task_cred() access

    commit c69e8d9 (CRED: Use RCU to access another task's creds and to
    release a task's own creds) added non rcu_read_lock() protected access
    to task creds of the target task in set_prio_one().

    The comment above the function says:
    * - the caller must hold the RCU read lock

    The calling code in sys_setpriority does read_lock(&tasklist_lock) but
    not rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.
    With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tick
    interrupt when they see no read side critical section.

    There is another instance of __task_cred() in sys_setpriority() itself
    which is equally unprotected.

    Wrap the whole code section into a rcu read side critical section to
    fix this quick and dirty.

    Will be revisited in course of the read_lock(&tasklist_lock) -> rcu
    crusade.

    Oleg noted further:

    This also fixes another bug here. find_task_by_vpid() is not safe
    without rcu_read_lock(). I do not mean it is not safe to use the
    result, just find_pid_ns() by itself is not safe.

    Usually tasklist gives enough protection, but if copy_process() fails
    it calls free_pid() lockless and does call_rcu(delayed_put_pid().
    This means, without rcu lock find_pid_ns() can't scan the hash table
    safely.

    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    LKML-Reference: <20091210004703.029784964@linutronix.de>
    Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    ---
    kernel/sys.c | 2 ++
    1 files changed, 2 insertions(+), 0 deletions(-)

    diff --git a/kernel/sys.c b/kernel/sys.c
    index 9968c5f..bc1dc61 100644
    --- a/kernel/sys.c
    +++ b/kernel/sys.c
    @@ -163,6 +163,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
    if (niceval > 19)
    niceval = 19;

    + rcu_read_lock();
    read_lock(&tasklist_lock);
    switch (which) {
    case PRIO_PROCESS:
    @@ -200,6 +201,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
    }
    out_unlock:
    read_unlock(&tasklist_lock);
    + rcu_read_unlock();
    out:
    return error;
    }

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