From: Jann Horn jannh@google.com
commit a7a74d7ff55a0c657bc46238b050460b9eacea95 upstream.
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 | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-)
--- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -681,17 +681,17 @@ int binder_alloc_mmap_handler(struct bin struct binder_buffer *buffer;
mutex_lock(&binder_alloc_mmap_lock); - if (alloc->buffer) { + if (alloc->buffer_size) { ret = -EBUSY; failure_string = "already mapped"; goto err_already_mapped; } + alloc->buffer_size = min_t(unsigned long, vma->vm_end - vma->vm_start, + SZ_4M); + mutex_unlock(&binder_alloc_mmap_lock);
alloc->buffer = (void __user *)vma->vm_start; - mutex_unlock(&binder_alloc_mmap_lock);
- alloc->buffer_size = min_t(unsigned long, vma->vm_end - vma->vm_start, - SZ_4M); alloc->pages = kcalloc(alloc->buffer_size / PAGE_SIZE, sizeof(alloc->pages[0]), GFP_KERNEL); @@ -722,8 +722,9 @@ err_alloc_buf_struct_failed: kfree(alloc->pages); alloc->pages = NULL; err_alloc_pages_failed: - mutex_lock(&binder_alloc_mmap_lock); alloc->buffer = NULL; + mutex_lock(&binder_alloc_mmap_lock); + alloc->buffer_size = 0; err_already_mapped: mutex_unlock(&binder_alloc_mmap_lock); binder_alloc_debug(BINDER_DEBUG_USER_ERROR,