Re: [PATCH] sys_getppid oopses on debug kernel

From: Dave Hansen
Date: Tue Aug 08 2006 - 11:24:57 EST

On Tue, 2006-08-08 at 14:22 +0400, Kirill Korotaev wrote:
> sys_getppid() optimization can access a freed memory.
> On kernels with DEBUG_SLAB turned ON, this results in
> Oops.
> +#else
> + /*
> + * ->real_parent could be released before dereference and
> + * we accessed freed kernel memory, which faults with debugging on.
> + * Keep it simple and stupid.
> + */
> + read_lock(&tasklist_lock);
> + pid = current->group_leader->real_parent->tgid;
> + read_unlock(&tasklist_lock);
> +#endif
> return pid;
> }

Accessing freed memory is a bug, always, not just *only* when slab
debugging is on, right? Doesn't this mean we could get junk, or that
the reader could potentially run off a bad pointer?

It seems that this patch only papers over the problem in the case when
it is observed, but doesn't really even fix the normal case.

Could we use a seqlock to determine when real_parent is in flux, and
re-read real_parent until we get a consistent one? We could use in in
lieu of the existing for() loop.

-- Dave

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