binder: validate alloc->mm in ->mmap() handler

Since commit 1da52815 ("binder: fix alloc->vma_vm_mm null-ptr
dereference") binder caches a pointer to the current->mm during open().
This fixes a null-ptr dereference reported by syzkaller. Unfortunately,
it also opens the door for a process to update its mm after the open(),
(e.g. via execve) making the cached alloc->mm pointer invalid.

Things get worse when the process continues to mmap() a vma. From this
point forward, binder will attempt to find this vma using an obsolete
alloc->mm reference. Such as in binder_update_page_range(), where the
wrong vma is obtained via vma_lookup(), yet binder proceeds to happily
insert new pages into it.

To avoid this issue fail the ->mmap() callback if we detect a mismatch
between the vma->vm_mm and the original alloc->mm pointer. This prevents
alloc->vm_addr from getting set, so that any subsequent vma_lookup()
calls fail as expected.

Fixes: 1da52815 ("binder: fix alloc->vma_vm_mm null-ptr dereference")
Reported-by: Jann Horn <jannh@google.com>
Cc: <stable@vger.kernel.org> # 5.15+
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Acked-by: Todd Kjos <tkjos@google.com>
Link: https://lore.kernel.org/r/20221104231235.348958-1-cmllamas@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder_alloc.c

@@ -739,6 +739,12 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
 	const char *failure_string;
 	struct binder_buffer *buffer;
 
+	if (unlikely(vma->vm_mm != alloc->mm)) {
+		ret = -EINVAL;
+		failure_string = "invalid vma->vm_mm";
+		goto err_invalid_mm;
+	}
+
 	mutex_lock(&binder_alloc_mmap_lock);
 	if (alloc->buffer_size) {
 		ret = -EBUSY;
@@ -785,6 +791,7 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
 	alloc->buffer_size = 0;
 err_already_mapped:
 	mutex_unlock(&binder_alloc_mmap_lock);
+err_invalid_mm:
 	binder_alloc_debug(BINDER_DEBUG_USER_ERROR,
 			   "%s: %d %lx-%lx %s failed %d\n", __func__,
 			   alloc->pid, vma->vm_start, vma->vm_end,