Oleg Nesterov <[email protected]> writes:
> On 08/16, Eric W. Biederman wrote:
>> Oleg Nesterov <[email protected]> writes:
>>
>> > On 08/15, Eric W. Biederman wrote:
>> >>
>> >> +static inline pid_t pid_nr(struct pid *pid)
>> >> +{
>> >> + pid_t nr = 0;
>> >> + if (pid)
>> >> + nr = pid->nr;
>> >> + return nr;
>> >> +}
>> >
>> > I think this is not safe, you need rcu locks here or the caller should
>> > do some locking.
>> >
>> > Let's look at f_getown() (PATCH 7/7). What if original task which was
>> > pointed by ->f_owner.pid has gone, another thread does fcntl(F_SETOWN),
>> > and pid_nr() takes a preemtion after 'if (pid)'? In this case 'pid->nr'
>> > may follow a freed memory.
>>
>> This isn't an rcu reference. I hold a hard reference count on
>> the pid entry. So this should be safe.
>
> -static void f_modown(struct file *filp, unsigned long pid,
> +static void f_modown(struct file *filp, struct pid *pid, enum pid_type
> type,
> uid_t uid, uid_t euid, int force)
> {
> write_lock_irq(&filp->f_owner.lock);
> if (force || !filp->f_owner.pid) {
> - filp->f_owner.pid = pid;
> + put_pid(filp->f_owner.pid);
>
> This 'put_pid()' can actually free 'struct pid' if the task/group
> has already gone away. Another thread doing f_getown() can access
> a freed memory, no?
Good point. In that case it looks like I need to hold the f_owner.lock.
Something needs to serialize that.
Fun. I touch the code and find a place where we didn't take a lock
and accidentally relied on integer operations being atomic.
I will see about working up a fix for that.
Eric
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