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 88021166 ("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: 88021166 ("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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder_alloc.c

@@ -680,17 +680,17 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
 	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);
@@ -721,8 +721,9 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
 	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,