Re: [PATCH RFC] pid: make setpgid() system call use RCU read-sidecritical section
From: Paul E. McKenney
Date: Mon Aug 30 2010 - 16:32:46 EST
On Mon, Aug 30, 2010 at 09:51:07PM +0200, Jiri Slaby wrote:
> Ccing Oleg.
>
> On 08/30/2010 07:26 PM, Paul E. McKenney wrote:
> > [ 23.584720] ===================================================
> > [ 23.585059] [ INFO: suspicious rcu_dereference_check() usage. ]
> > [ 23.585176] ---------------------------------------------------
> > [ 23.585176] kernel/pid.c:419 invoked rcu_dereference_check() without protection!
> > [ 23.585176]
> > [ 23.585176] other info that might help us debug this:
> > [ 23.585176]
> > [ 23.585176]
> > [ 23.585176] rcu_scheduler_active = 1, debug_locks = 1
> > [ 23.585176] 1 lock held by rc.sysinit/728:
> > [ 23.585176] #0: (tasklist_lock){.+.+..}, at: [<ffffffff8104771f>] sys_setpgid+0x5f/0x193
> > [ 23.585176]
> > [ 23.585176] stack backtrace:
> > [ 23.585176] Pid: 728, comm: rc.sysinit Not tainted 2.6.36-rc2 #2
> > [ 23.585176] Call Trace:
> > [ 23.585176] [<ffffffff8105b436>] lockdep_rcu_dereference+0x99/0xa2
> > [ 23.585176] [<ffffffff8104c324>] find_task_by_pid_ns+0x50/0x6a
> > [ 23.585176] [<ffffffff8104c35b>] find_task_by_vpid+0x1d/0x1f
> > [ 23.585176] [<ffffffff81047727>] sys_setpgid+0x67/0x193
> > [ 23.585176] [<ffffffff810029eb>] system_call_fastpath+0x16/0x1b
> > [ 24.959669] type=1400 audit(1282938522.956:4): avc: denied { module_request } for pid=766 comm="hwclock" kmod="char-major-10-135" scontext=system_u:system_r:hwclock_t:s0 tcontext=system_u:system_r:kernel_t:s0 tclas
> >
> > It turns out that the setpgid() system call fails to enter an RCU
> > read-side critical section before doing a PID-to-task_struct translation.
> > This commit therefore does rcu_read_lock() before the translation, and
> > also does rcu_read_unlock() after the last use of the returned pointer.
> >
> > Located-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > ---
> >
> > sys.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index e9ad444..05a4b0c 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -938,6 +938,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
> > write_lock_irq(&tasklist_lock);
> >
> > err = -ESRCH;
> > + rcu_read_lock();
> > p = find_task_by_vpid(pid);
>
> AFAICT the missing lock doesn't harm due to the write_lock of tasklist
> above. But is probably a good thing to do anyway.
Or we can add the tasklist lock to the rcu_dereference_check() condition.
Thanx, Paul
> regards,
> --
> js
> suse labs
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/