Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Wed, 9 Dec 2009 17:25:17 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [patch 1/9] sys: Fix missing rcu protection for __task_cred() access |
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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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