UPSTREAM: futex: Validate atomic acquisition in futex_lock_pi_atomic()

We need to protect the atomic acquisition in the kernel against rogue
user space which sets the user space futex to 0, so the kernel side
acquisition succeeds while there is existing state in the kernel
associated to the real owner.

Verify whether the futex has waiters associated with kernel state. If
it has, return -EINVAL. The state is corrupted already, so no point in
cleaning it up. Subsequent calls will fail as well. Not our problem.

[ tglx: Use futex_top_waiter() and explain why we do not need to try
  	restoring the already corrupted user space state. ]

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Drewry <wad@chromium.org>
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

BUG=chromium:377392
TEST=nyan build & boot

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/202665
Reviewed-by: Will Drewry <wad@chromium.org>

(cherry picked from ToT commit bfc19d26e4fa90045c0084cdc130d185f972e84a)
Signed-off-by: Kees Cook <keescook@chromium.org>

Change-Id: I42b9c2c2f7776be260ad2197bc1ac4287e021007
Reviewed-on: https://chromium-review.googlesource.com/202931
Tested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Will Drewry <wad@chromium.org>
Commit-Queue: Kees Cook <keescook@chromium.org>
1 file changed