Re: [PATCH v3] binder: add mutex_lock for mmap and NULL when free

[Kassey Li]

On 2023/10/8 7:16, Carlos Llamas wrote:
> On Sat, Oct 07, 2023 at 08:28:13PM +0800, Kassey Li wrote:
> > - Enforce alloc->mutex in binder_alloc_mmap_handler when add the entry to
> >    list.
> >
> > - Assign the freed pages/page_ptr to NULL to catch possible use after free
> >    with NULL pointer access.
> >
> > Fixes: 19c987241ca1 ("binder: separate out binder_alloc functions")
> > Fixes: f2517eb76f1f ("android: binder: Add global lru shrinker to binder")
> > CC: Todd Kjos <tkjos@xxxxxxxxxxx>
> > CC: Sherry Yang <sherryy@xxxxxxxxxxx>
> > Link: https://lore.kernel.org/all/20231007034046.2352124-1-quic_yingangl@xxxxxxxxxxx/
> > Signed-off-by: Kassey Li <quic_yingangl@xxxxxxxxxxx>
> > ---
> > V1 -> V2: Add Fixes info.
> > V2 -> V3: Add this history.
> > ---
> >   drivers/android/binder_alloc.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> > index e3db8297095a..c7d126e04343 100644
> > --- a/drivers/android/binder_alloc.c
> > +++ b/drivers/android/binder_alloc.c
> > @@ -775,6 +775,7 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
> >   	}
> >   
> >   	buffer->user_data = alloc->buffer;
> > +	mutex_lock(&alloc->mutex);
>
> At this stage we are already holding the mm->mmap_lock. If you take the
> alloc->mutex here you will deadlock. You should see this warning with
> lockdep enabled. Also, you don't need the lock here...

  lockdep did not warn, but i agree with you, i did not aware the 
mm->mmap_lock. thanks for the remind.
> >   	list_add(&buffer->entry, &alloc->buffers);
> >   	buffer->free = 1;
> >   	binder_insert_free_buffer(alloc, buffer);
> > @@ -782,7 +783,7 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
> >   
> >   	/* Signal binder_alloc is fully initialized */
> >   	binder_alloc_set_vma(alloc, vma);
>
> This barrier will set the alloc->vma pointer. Once set, it signals that
> the alloc-> space has been initialized and it is safe to access.
>
> > -
> > +	mutex_unlock(&alloc->mutex);
> >   	return 0;
> >   
> >   err_alloc_buf_struct_failed:
> > @@ -856,9 +857,11 @@ void binder_alloc_deferred_release(struct binder_alloc *alloc)
> >   				     __func__, alloc->pid, i, page_addr,
> >   				     on_lru ? "on lru" : "active");
> >   			__free_page(alloc->pages[i].page_ptr);
> > +			alloc->pages[i].page_ptr = NULL;
> >   			page_count++;
> >   		}
> >   		kfree(alloc->pages);
> > +		alloc->pages = NULL;
>
> The process is dying and there aren't any more references to it. It is
> pointless to mark the pages NULL. No one is or will use them after.

  looks correct.
> >   	}
> >   	mutex_unlock(&alloc->mutex);
> >   	if (alloc->mm)
> > --
> > 2.25.1
>
> I see that you reported a crash on the previous thread here:
> https://lore.kernel.org/all/26988068-8c9f-8591-db6e-44c8105af638@xxxxxxxxxxx/
> ...unfortunately, it doesn't seem to me like setting alloc->pages = NULL
> would help fix your issue.
>
> I do agree this sounds like a use-after-free though. Are you able to
> reproduce the issue with KASAN enabled? This should tell us where the
> actual problem is.

  kasan is trying but not reproduced yet,  we will chasing with this 
test enable kasan.
> --
> Carlos Llamas