Messages in this thread | | | Date | Mon, 24 Sep 2007 10:18:34 +0200 | From | Nadia Derbey <> | Subject | Re: 2.6.23-rc6-mm1: IPC: sleeping function called ... |
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Jarek Poplawski wrote: > On Fri, Sep 21, 2007 at 01:03:47PM +0200, Jarek Poplawski wrote: > ... > >>I hope not! But, then it would be probably another logical trick: >>ipc_rcu_getref/putref() seems to prevent kfreeing of a structure, so >>if it's used in do_msgsnd() there should be a risk something can do >>this kfree at this moment, and it seems freeque() is the only one, >>which both: can do this and cares for this refcount. Then, e.g., if >>any of them does ipc_rcu_getref() a bit later and sees old (cached) >>value - kfree can be skipped forever. [...] > > > After rethinking, this scenario seems to be wrong or very unprobable > (I'm not sure of all ways "if (--container...)" could be compiled), > so there should be no such risk - double kfree/vfree is more probable, > so no danger. More likely is such refcount abuse: ipc_rcu_getref() in > do_msgsnd() done a bit after ipc_rcu_putref() in freeque() (msq > pointer acquired by do_msgsend() before freeque() started); then, > after schedule(), do_msgsnd() can work with kfreed msq_queue structure > (at least considering classic RCU). >
If msgsnd() acquires the pointer first, it does it under lock + rcu_getref(). ==> refcount = 2 When schedule() is called if freeque() takes the pointer it will call msg_rmid() that sets the deleted field in the msg queue. When the lock is released by freeque(), we have either 1) or 2): 1) freeque()'s putref called 1st ==> refocunt = 1 Then msgsnd()'s lock_by_ptr() is called ==> rcu lock Then msgsnd()'s putref is called ==> refcount = 0 But this is done under RCU lock, so should be no problem Then the deleted field is checked ==> return 2) msgsnd()'s lock_by_ptr() is called ==> rcu lock Then we don't mind in which order are done the other operations since we under rcu_lock: the structure won't disappear till we test the deleted field.
Regards, Nadia
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