linux-kernel - [PATCH] afs: Fix race in commit bulk status fetch

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Date:   Thu, 14 Nov 2019 18:41:03 +0000
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org
Cc:     dhowells@...hat.com, marc.dionne@...istor.com,
        linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] afs: Fix race in commit bulk status fetch

When a lookup is done, the afs filesystem will perform a bulk status-fetch
operation on the requested vnode (file) plus the next 49 other vnodes from
the directory list (in AFS, directory contents are downloaded as blobs and
parsed locally).  When the results are received, it will speculatively
populate the inode cache from the extra data.

However, if the lookup races with another lookup on the same directory, but
for a different file - one that's in the 49 extra fetches, then if the bulk
status-fetch operation finishes first, it will try and update the inode
from the other lookup.

If this other inode is still in the throes of being created, however, this
will cause an assertion failure in afs_apply_status():

	BUG_ON(test_bit(AFS_VNODE_UNSET, &vnode->flags));

on or about fs/afs/inode.c:175 because it expects data to be there already
that it can compare to.

Fix this by skipping the update if the inode is being created as the
creator will presumably set up the inode with the same information.

Fixes: 39db9815da48 ("afs: Fix application of the results of a inline bulk status fetch")
Signed-off-by: David Howells <dhowells@...hat.com>
Reviewed-by: Marc Dionne <marc.dionne@...istor.com>
---

 fs/afs/dir.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index cc12772d0a4d..497f979018c2 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -803,7 +803,12 @@ static struct inode *afs_do_lookup(struct inode *dir, struct dentry *dentry,
 			continue;

 		if (cookie->inodes[i]) {
-			afs_vnode_commit_status(&fc, AFS_FS_I(cookie->inodes[i]),
+			struct afs_vnode *iv = AFS_FS_I(cookie->inodes[i]);
+
+			if (test_bit(AFS_VNODE_UNSET, &iv->flags))
+				continue;
+
+			afs_vnode_commit_status(&fc, iv,
 						scb->cb_break, NULL, scb);
 			continue;
 		}