lkml.org 
[lkml]   [2023]   [Nov]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH 03/21] binder: fix race between mmput() and do_exit()
From
Carlos Llamas <cmllamas@google.com> writes:
> 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")
> Signed-off-by: Carlos Llamas <cmllamas@google.com>

Nice catch!

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

The code really could use a comment, though.

Alice

\
 
 \ /
  Last update: 2023-11-20 13:51    [W:8.449 / U:0.008 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site