binder: Prevent repeated use of ->mmap() via NULL mapping
authorJann Horn <jannh@google.com>
Fri, 18 Oct 2019 20:56:30 +0000 (22:56 +0200)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 14 Nov 2019 03:44:47 +0000 (11:44 +0800)
commita7a74d7ff55a0c657bc46238b050460b9eacea95
tree37be4c7f15bfa4535a2aa0a3a1a28ee58d10627f
parent8eb52a1ee37aafd9b796713aa0b3ab9cbc455be3
binder: Prevent repeated use of ->mmap() via NULL mapping

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@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191018205631.248274-2-jannh@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/android/binder_alloc.c