android: binder: stop saving a pointer to the VMA

Do not record a pointer to a VMA outside of the mmap_lock for later use. 
This is unsafe and there are a number of failure paths *after* the
recorded VMA pointer may be freed during setup.  There is no callback to
the driver to clear the saved pointer from generic mm code.  Furthermore,
the VMA pointer may become stale if any number of VMA operations end up
freeing the VMA so saving it was fragile to being with.

Instead, change the binder_alloc struct to record the start address of the
VMA and use vma_lookup() to get the vma when needed.  Add lockdep
mmap_lock checks on updates to the vma pointer to ensure the lock is held
and depend on that lock for synchronization of readers and writers - which
was already the case anyways, so the smp_wmb()/smp_rmb() was not
necessary.

[akpm@linux-foundation.org: fix drivers/android/binder_alloc_selftest.c]
Link: https://lkml.kernel.org/r/20220621140212.vpkio64idahetbyf@revolver
Fixes: da1b956 ("android: binder: fix the race mmap and alloc_new_buf_locked")
Reported-by: syzbot+58b51ac2b04e388ab7b0@syzkaller.appspotmail.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Christian Brauner (Microsoft) <brauner@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hridya Valsaraju <hridya@google.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Martijn Coenen <maco@android.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Todd Kjos <tkjos@android.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/android/binder_alloc.c

@@ -213,7 +213,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 
 	if (mm) {
 		mmap_read_lock(mm);
-		vma = alloc->vma;
+		vma = vma_lookup(mm, alloc->vma_addr);
 	}
 
 	if (!vma && need_mm) {
@@ -313,28 +313,25 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 static inline void binder_alloc_set_vma(struct binder_alloc *alloc,
 		struct vm_area_struct *vma)
 {
-	if (vma)
+	unsigned long vm_start = 0;
+
+	if (vma) {
+		vm_start = vma->vm_start;
 		alloc->vma_vm_mm = vma->vm_mm;
-	/*
-	 * If we see alloc->vma is not NULL, buffer data structures set up
-	 * completely. Look at smp_rmb side binder_alloc_get_vma.
-	 * We also want to guarantee new alloc->vma_vm_mm is always visible
-	 * if alloc->vma is set.
-	 */
-	smp_wmb();
-	alloc->vma = vma;
+	}
+
+	mmap_assert_write_locked(alloc->vma_vm_mm);
+	alloc->vma_addr = vm_start;
 }
 
 static inline struct vm_area_struct *binder_alloc_get_vma(
 		struct binder_alloc *alloc)
 {
 	struct vm_area_struct *vma = NULL;
 
-	if (alloc->vma) {
-		/* Look at description in binder_alloc_set_vma */
-		smp_rmb();
-		vma = alloc->vma;
-	}
+	if (alloc->vma_addr)
+		vma = vma_lookup(alloc->vma_vm_mm, alloc->vma_addr);
+
 	return vma;
 }
 
@@ -817,7 +814,8 @@ void binder_alloc_deferred_release(struct binder_alloc *alloc)
 
 	buffers = 0;
 	mutex_lock(&alloc->mutex);
-	BUG_ON(alloc->vma);
+	BUG_ON(alloc->vma_addr &&
+	       vma_lookup(alloc->vma_vm_mm, alloc->vma_addr));
 
 	while ((n = rb_first(&alloc->allocated_buffers))) {
 		buffer = rb_entry(n, struct binder_buffer, rb_node);

drivers/android/binder_alloc.h

@@ -100,7 +100,7 @@ struct binder_lru_page {
  */
 struct binder_alloc {
 	struct mutex mutex;
-	struct vm_area_struct *vma;
+	unsigned long vma_addr;
 	struct mm_struct *vma_vm_mm;
 	void __user *buffer;
 	struct list_head buffers;

drivers/android/binder_alloc_selftest.c

@@ -287,7 +287,7 @@ void binder_selftest_alloc(struct binder_alloc *alloc)
 	if (!binder_selftest_run)
 		return;
 	mutex_lock(&binder_selftest_lock);
-	if (!binder_selftest_run || !alloc->vma)
+	if (!binder_selftest_run || !alloc->vma_addr)
 		goto done;
 	pr_info("STARTED\n");
 	binder_selftest_alloc_offset(alloc, end_offset, 0);