[PATCH] nfs/localio: fix regression due to out-of-order __put_cred [was: Re: [Bug][xfstests crash on nfs] kernel BUG at kernel/cred.c:104!]

[Mike Snitzer <snitzer@kernel.org>]

Commit 86855311c117 ("nfs/localio: add refcounting for each iocb IO
associated with NFS pgio header") inadvertantly reintroduced the same
potential for __put_cred() triggering BUG_ON(cred == current->cred)
that commit 992203a1fba5 ("nfs/localio: restore creds before releasing
pageio data") fixed.

Fix this by saving and restoring the cred around each {read,write}_iter 
call within the respective for loop of nfs_local_call_{read,write}.

Reported-by: Zorro Lang <zlang@redhat.com>
Fixes: 86855311c117 ("nfs/localio: add refcounting for each iocb IO associated with NFS pgio header")
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

diff --git a/fs/nfs/localio.c b/fs/nfs/localio.c
index 6461ce3ba9a4..eeb05a0d8d26 100644
--- a/fs/nfs/localio.c
+++ b/fs/nfs/localio.c
@@ -623,8 +623,6 @@ static void nfs_local_call_read(struct work_struct *work)
 	ssize_t status;
 	int n_iters;
 
-	save_cred = override_creds(filp->f_cred);
-
 	n_iters = atomic_read(&iocb->n_iters);
 	for (int i = 0; i < n_iters ; i++) {
 		if (iocb->iter_is_dio_aligned[i]) {
@@ -637,7 +635,10 @@ static void nfs_local_call_read(struct work_struct *work)
 		} else
 			iocb->kiocb.ki_flags &= ~IOCB_DIRECT;
 
+		save_cred = override_creds(filp->f_cred);
 		status = filp->f_op->read_iter(&iocb->kiocb, &iocb->iters[i]);
+		revert_creds(save_cred);
+
 		if (status != -EIOCBQUEUED) {
 			if (unlikely(status >= 0 && status < iocb->iters[i].count))
 				force_done = true; /* Partial read */
@@ -647,8 +648,6 @@ static void nfs_local_call_read(struct work_struct *work)
 			}
 		}
 	}
-
-	revert_creds(save_cred);
 }
 
 static int
@@ -830,7 +829,6 @@ static void nfs_local_call_write(struct work_struct *work)
 	int n_iters;
 
 	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
-	save_cred = override_creds(filp->f_cred);
 
 	file_start_write(filp);
 	n_iters = atomic_read(&iocb->n_iters);
@@ -845,7 +843,10 @@ static void nfs_local_call_write(struct work_struct *work)
 		} else
 			iocb->kiocb.ki_flags &= ~IOCB_DIRECT;
 
+		save_cred = override_creds(filp->f_cred);
 		status = filp->f_op->write_iter(&iocb->kiocb, &iocb->iters[i]);
+		revert_creds(save_cred);
+
 		if (status != -EIOCBQUEUED) {
 			if (unlikely(status >= 0 && status < iocb->iters[i].count))
 				force_done = true; /* Partial write */
@@ -857,7 +858,6 @@ static void nfs_local_call_write(struct work_struct *work)
 	}
 	file_end_write(filp);
 
-	revert_creds(save_cred);
 	current->flags = old_flags;
 }