padata: Always leave BHs disabled when running ->parallel()

A deadlock can happen when an overloaded system runs ->parallel() in the
context of the current task:

    padata_do_parallel
      ->parallel()
        pcrypt_aead_enc/dec
          padata_do_serial
            spin_lock(&reorder->lock) // BHs still enabled
              <interrupt>
                ...
                  __do_softirq
                    ...
                      padata_do_serial
                        spin_lock(&reorder->lock)

It's a bug for BHs to be on in _do_serial as Steffen points out, so
ensure they're off in the "current task" case like they are in
padata_parallel_worker to avoid this situation.

Reported-by: syzbot+bc05445bc14148d51915@syzkaller.appspotmail.com
Fixes: 4611ce224688 ("padata: allocate work structures for parallel jobs from a pool")
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/kernel/padata.c b/kernel/padata.c
index e5819bb8bd1dcc9bc4903816fcd29808bb8166ac..97f51e0c1776677d65a7f3403a06eba87f672aa6 100644 (file)
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -207,14 +207,16 @@ int padata_do_parallel(struct padata_shell *ps,
        pw = padata_work_alloc();
        spin_unlock(&padata_works_lock);
 
+       if (!pw) {
+               /* Maximum works limit exceeded, run in the current task. */
+               padata->parallel(padata);
+       }
+
        rcu_read_unlock_bh();
 
        if (pw) {
                padata_work_init(pw, padata_parallel_worker, padata, 0);
                queue_work(pinst->parallel_wq, &pw->pw_work);
-       } else {
-               /* Maximum works limit exceeded, run in the current task. */
-               padata->parallel(padata);
        }
 
        return 0;