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    SubjectRe: [patch 1/9] sys: Fix missing rcu protection for __task_cred() access
    On Thu, Dec 10, 2009 at 12:52:51AM -0000, Thomas Gleixner wrote:
    > 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.

    And this is called out in Documentation/RCU/checklist.txt, item #2:

    2. Do the RCU read-side critical sections make proper use of
    rcu_read_lock() and friends? These primitives are needed
    to prevent grace periods from ending prematurely, which
    could result in data being unceremoniously freed out from
    under your read-side code, which can greatly increase the
    actuarial risk of your kernel.

    As a rough rule of thumb, any dereference of an RCU-protected
    pointer must be covered by rcu_read_lock() or rcu_read_lock_bh()
    or by the appropriate update-side lock.

    > 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.

    Good catch!

    Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    > 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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