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    Subject[patch 1/9] 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.

    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: David Howells <dhowells@redhat.com>
    Cc: James Morris <jmorris@namei.org>
    Cc: linux-security-module@vger.kernel.org
    ---
    kernel/sys.c | 2 ++
    1 file changed, 2 insertions(+)

    Index: linux-2.6-tip/kernel/sys.c
    ===================================================================
    --- linux-2.6-tip.orig/kernel/sys.c
    +++ linux-2.6-tip/kernel/sys.c
    @@ -163,6 +163,7 @@ SYSCALL_DEFINE3(setpriority, int, which,
    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,
    }
    out_unlock:
    read_unlock(&tasklist_lock);
    + rcu_read_unlock();
    out:
    return error;
    }



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