Re: [PATCH 2/3] binder: Prevent repeated use of ->mmap() via NULL mapping

[Christian Brauner]

On Fri Oct 18, 2019 at 10:56 PM Jann Horn wrote:
> binder_alloc_mmap_handler() attempts to detect the use of ->mmap() on a
> binder_proc whose binder_alloc has already been initialized by checking
> whether alloc->buffer is non-zero.
> 
> Before commit 880211667b20 ("binder: remove kernel vm_area for buffer
> space"), alloc->buffer was a kernel mapping address, which is always
> non-zero, but since that commit, it is a userspace mapping address.
> 
> A sufficiently privileged user can map /dev/binder at NULL, tricking
> binder_alloc_mmap_handler() into assuming that the binder_proc has not been
> mapped yet. This leads to memory unsafety.
> Luckily, no context on Android has such privileges, and on a typical Linux
> desktop system, you need to be root to do that.
> 
> Fix it by using the mapping size instead of the mapping address to
> distinguish the mapped case. A valid VMA can't have size zero.
> 
> Fixes: 880211667b20 ("binder: remove kernel vm_area for buffer space")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>

Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>