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

[Greg Kroah-Hartman]

On Fri, Aug 31, 2018 at 01:17:20PM -0700, Greg Hackmann wrote:
> On 08/31/2018 01:12 PM, Greg Kroah-Hartman wrote:
> > 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.
> > 
> > thanks,
> > 
> > greg k-h
> > 
> 
> Yes, 4.4.y and older will need this.  If there are no objections to this
> patch, I'll send backports ASAP.

None from me, thanks!

greg k-h