Re: [PATCH] staging: android: ion: fix ION_IOC_{MAP,SHARE} use-after-free

From: Greg Kroah-Hartman
Date: Fri Aug 31 2018 - 16:12:36 EST

On Fri, Aug 31, 2018 at 01:06:27PM -0700, Greg Hackmann wrote:
> The ION_IOC_{MAP,SHARE} ioctls drop and reacquire client->lock several
> times while operating on one of the client's ion_handles. This creates
> windows where userspace can call ION_IOC_FREE on the same client with
> the same handle, and effectively make the kernel drop its own reference.
> For example:
> - thread A: ION_IOC_ALLOC creates an ion_handle with refcount 1
> - thread A: starts ION_IOC_MAP and increments the refcount to 2
> - thread B: ION_IOC_FREE decrements the refcount to 1
> - thread B: ION_IOC_FREE decrements the refcount to 0 and frees the
> handle
> - thread A: continues ION_IOC_MAP with a dangling ion_handle * to
> freed memory
> Fix this by holding client->lock for the duration of
> ION_IOC_{MAP,SHARE}, preventing the concurrent ION_IOC_FREE. Also
> remove ion_handle_get_by_id(), since there's literally no way to use it
> safely.
> This patch is applied on top of 4.9.y. Kernels 4.12 and later are
> unaffected, since all the underlying ion_handle infrastructure has been
> ripped out.

Does 4.4.y or older also need this? If so, can you send backports, as
this one does not apply there.


greg k-h