linux-kernel - [PATCH] nfsd: Close a race between access checking/setting in nfs4_get_vfs

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Date:	Thu,  9 Jun 2016 17:01:39 -0400
From:	Oleg Drokin <green@...uxhacker.ru>
To:	Jeff Layton <jlayton@...chiereds.net>,
	"J . Bruce Fields" <bfields@...ldses.org>
Cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Oleg Drokin <green@...uxhacker.ru>
Subject: [PATCH] nfsd: Close a race between access checking/setting in nfs4_get_vfs_file

Currently there's an unprotected access mode check in nfs4_upgrade_open
that then calls nfs4_get_vfs_file which in turn assumes whatever
access mode was present in the state is still valid which is racy.
Two nfs4_get_vfs_file van enter the same path as result and get two
references to nfs4_file, but later drop would only happens once because
access mode is only denoted by bits, so no refcounting.

The locking around access mode testing is introduced to avoid this race.

Signed-off-by: Oleg Drokin <green@...uxhacker.ru>
---

This patch performs equally well to the st_rwsem -> mutex conversion,
but is a bit ligher-weight I imagine.
For one it seems to allow truncates in parallel if we ever want it.

 fs/nfsd/nfs4state.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index f5f82e1..d4b9eba 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -3958,6 +3958,11 @@ static __be32 nfs4_get_vfs_file(struct svc_rqst *rqstp, struct nfs4_file *fp,
 
 	spin_lock(&fp->fi_lock);
 
+	if (test_access(open->op_share_access, stp)) {
+		spin_unlock(&fp->fi_lock);
+		return nfserr_eagain;
+	}
+
 	/*
 	 * Are we trying to set a deny mode that would conflict with
 	 * current access?
@@ -4017,11 +4022,21 @@ nfs4_upgrade_open(struct svc_rqst *rqstp, struct nfs4_file *fp, struct svc_fh *c
 	__be32 status;
 	unsigned char old_deny_bmap = stp->st_deny_bmap;
 
-	if (!test_access(open->op_share_access, stp))
-		return nfs4_get_vfs_file(rqstp, fp, cur_fh, stp, open);
+again:
+	spin_lock(&fp->fi_lock);
+	if (!test_access(open->op_share_access, stp)) {
+		spin_unlock(&fp->fi_lock);
+		status = nfs4_get_vfs_file(rqstp, fp, cur_fh, stp, open);
+		/*
+		 * Somebody won the race for access while we did not hold
+		 * the lock here
+		 */
+		if (status == nfserr_eagain)
+			goto again;
+		return status;
+	}
 
 	/* test and set deny mode */
-	spin_lock(&fp->fi_lock);
 	status = nfs4_file_check_deny(fp, open->op_share_deny);
 	if (status == nfs_ok) {
 		set_deny(open->op_share_deny, stp);
@@ -4361,6 +4376,13 @@ nfsd4_process_open2(struct svc_rqst *rqstp, struct svc_fh *current_fh, struct nf
 		status = nfs4_get_vfs_file(rqstp, fp, current_fh, stp, open);
 		if (status) {
 			up_read(&stp->st_rwsem);
+			/*
+			 * EAGAIN is returned when there's a racing access,
+			 * this should never happen as we are the only user
+			 * of this new state, and since it's not yet hashed,
+			 * nobody can find it
+			 */
+			WARN_ON(status == nfserr_eagain);
 			release_open_stateid(stp);
 			goto out;
 		}
-- 
2.7.4