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
