android: binder: stop saving a pointer to the VMA

[upstream commit a43cfc8]

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.

[[email protected]: 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: [email protected]
Signed-off-by: Liam R. Howlett <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Christian Brauner (Microsoft) <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Hridya Valsaraju <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Martijn Coenen <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Todd Kjos <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

drivers/android/binder_alloc.c

@@ -317,9 +317,10 @@ static inline void binder_alloc_set_vma(struct binder_alloc *alloc,
 
 	if (vma) {
 		vm_start = vma->vm_start;
-		mmap_assert_write_locked(alloc->vma_vm_mm);
+		alloc->vma_vm_mm = vma->vm_mm;
 	}
 
+	mmap_assert_write_locked(alloc->vma_vm_mm);
 	alloc->vma_addr = vm_start;
 }