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

[Carlos Llamas]

On Fri, Jan 19, 2024 at 06:48:43AM +0100, Greg Kroah-Hartman wrote:
> 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>

Oops, here is the complete info:

Commit ID: 9a9ab0d963621d9d12199df9817e66982582d5a5
Subject:   "binder: fix race between mmput() and do_exit()"
Reason:    Fixes a race condition in binder.
Versions:  v4.19+

Note this will have a trivial conflict in v4.19 and v5.10 kernels as
commit d8ed45c5dcd4 is not there. Please let me know if I should send
those patches separately.

Thanks,
--
Carlos Llamas