commit fd2214199a952aec25440b8a73ac017e889adf3b
author Li Li <dualli@google.com> Tue Sep 07 16:12:53 2021 -0700
committer Li Li <dualli@google.com> Tue Sep 14 07:22:07 2021 +0000
tree 2be0a6a2eef40e7a423c69bff6327110fe06d23d
parent fca745e32dfec052ecc04431f0ca58d49cce8e94

FROMGIT: binder: fix freeze race

Currently cgroup freezer is used to freeze the application threads, and
BINDER_FREEZE is used to freeze the corresponding binder interface.
There's already a mechanism in ioctl(BINDER_FREEZE) to wait for any
existing transactions to drain out before actually freezing the binder
interface.

But freezing an app requires 2 steps, freezing the binder interface with
ioctl(BINDER_FREEZE) and then freezing the application main threads with
cgroupfs. This is not an atomic operation. The following race issue
might happen.

1) Binder interface is frozen by ioctl(BINDER_FREEZE);
2) Main thread A initiates a new sync binder transaction to process B;
3) Main thread A is frozen by "echo 1 > cgroup.freeze";
4) The response from process B reaches the frozen thread, which will
unexpectedly fail.

This patch provides a mechanism to check if there's any new pending
transaction happening between ioctl(BINDER_FREEZE) and freezing the
main thread. If there's any, the main thread freezing operation can
be rolled back to finish the pending transaction.

Furthermore, the response might reach the binder driver before the
rollback actually happens. That will still cause failed transaction.

As the other process doesn't wait for another response of the response,
the response transaction failure can be fixed by treating the response
transaction like an oneway/async one, allowing it to reach the frozen
thread. And it will be consumed when the thread gets unfrozen later.

NOTE: This patch reuses the existing definition of struct
binder_frozen_status_info but expands the bit assignments of __u32
member sync_recv.

To ensure backward compatibility, bit 0 of sync_recv still indicates
there's an outstanding sync binder transaction. This patch adds new
information to bit 1 of sync_recv, indicating the binder transaction
happens exactly when there's a race.

If an existing userspace app runs on a new kernel, a sync binder call
will set bit 0 of sync_recv so ioctl(BINDER_GET_FROZEN_INFO) still
return the expected value (true). The app just doesn't check bit 1
intentionally so it doesn't have the ability to tell if there's a race.
This behavior is aligned with what happens on an old kernel which
doesn't set bit 1 at all.

A new userspace app can 1) check bit 0 to know if there's a sync binder
transaction happened when being frozen - same as before; and 2) check
bit 1 to know if that sync binder transaction happened exactly when
there's a race - a new information for rollback decision.

Fixes: 432ff1e91694 ("binder: BINDER_FREEZE ioctl")
Acked-by: Todd Kjos <tkjos@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Li Li <dualli@google.com>
Test: stress test with apps being frozen and initiating binder calls at
the same time, confirmed the pending transactions succeeded.
Link: https://lore.kernel.org/r/20210910164210.2282716-2-dualli@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Bug: 198493121
(cherry picked from commit b564171ade70570b7f335fa8ed17adb28409e3ac
 git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
 char-misc-linus)
Change-Id: I488ba75056f18bb3094ba5007027b76b5caebec9