Re: [PATCH v2 03/28] binder: fix race between mmput() and do_exit()

[Greg Kroah-Hartman]

On Thu, Jan 18, 2024 at 07:29:07PM +0000, Carlos Llamas wrote:
> On Fri, Dec 01, 2023 at 05:21:32PM +0000, Carlos Llamas wrote:
> > Task A calls binder_update_page_range() to allocate and insert pages on
> > a remote address space from Task B. For this, Task A pins the remote mm
> > via mmget_not_zero() first. This can race with Task B do_exit() and the
> > final mmput() refcount decrement will come from Task A.
> > 
> >   Task A            | Task B
> >   ------------------+------------------
> >   mmget_not_zero()  |
> >                     |  do_exit()
> >                     |    exit_mm()
> >                     |      mmput()
> >   mmput()           |
> >     exit_mmap()     |
> >       remove_vma()  |
> >         fput()      |
> > 
> > In this case, the work of ____fput() from Task B is queued up in Task A
> > as TWA_RESUME. So in theory, Task A returns to userspace and the cleanup
> > work gets executed. However, Task A instead sleep, waiting for a reply
> > from Task B that never comes (it's dead).
> > 
> > This means the binder_deferred_release() is blocked until an unrelated
> > binder event forces Task A to go back to userspace. All the associated
> > death notifications will also be delayed until then.
> > 
> > In order to fix this use mmput_async() that will schedule the work in
> > the corresponding mm->async_put_work WQ instead of Task A.
> > 
> > Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> > Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > Signed-off-by: Carlos Llamas <cmllamas@xxxxxxxxxx>
> > ---
> 
> Sorry, I forgot to Cc: stable@xxxxxxxxxxxxxxx.

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>