On Fri, Nov 4, 2022 at 4:12 PM Carlos Llamas cmllamas@google.com wrote:

Since commit 1da52815d5f1 ("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: 1da52815d5f1 ("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

---

drivers/android/binder_alloc.c | 7 +++++++ 1 file changed, 7 insertions(+)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c index 1c39cfce32fa..4ad42b0f75cd 100644 --- a/drivers/android/binder_alloc.c +++ b/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, -- 2.38.1.431.g37b22c650d-goog