linux-kernel - [PATCH] nfsd: Make creates return EEXIST correctly instead of EPERM

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Date:	Thu,  7 Jul 2016 21:47:46 -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: Make creates return EEXIST correctly instead of EPERM

It looks like we are bit overzealous about failing mkdir/create/mknod
with permission denied if the parent dir is not writeable.
Need to make sure the name does not exist first, because we need to
return EEXIST in that case.

Signed-off-by: Oleg Drokin <green@...uxhacker.ru>
---
A very similar problem exists with symlinks, but the patch is more
involved, so assuming this one is ok, I'll send a symlink one separately.
 fs/nfsd/nfs4proc.c |  6 +++++-
 fs/nfsd/vfs.c      | 11 ++++++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index de1ff1d..0067520 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -605,8 +605,12 @@ nfsd4_create(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 
 	fh_init(&resfh, NFS4_FHSIZE);
 
+	/*
+	 * We just check thta parent is accessible here, nfsd_* do their
+	 * own access permission checks
+	 */
 	status = fh_verify(rqstp, &cstate->current_fh, S_IFDIR,
-			   NFSD_MAY_CREATE);
+			   NFSD_MAY_EXEC);
 	if (status)
 		return status;
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 6fbd81e..6a45ec6 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1161,7 +1161,11 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	if (isdotent(fname, flen))
 		goto out;
 
-	err = fh_verify(rqstp, fhp, S_IFDIR, NFSD_MAY_CREATE);
+	/*
+	 * Even though it is a create, first we see if we are even allowed
+	 * to peek inside the parent
+	 */
+	err = fh_verify(rqstp, fhp, S_IFDIR, NFSD_MAY_EXEC);
 	if (err)
 		goto out;
 
@@ -1211,6 +1215,11 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
 		goto out; 
 	}
 
+	/* Now let's see if we actually have permissions to create */
+	err = nfsd_permission(rqstp, fhp->fh_export, dentry, NFSD_MAY_CREATE);
+	if (err)
+		goto out;
+
 	if (!(iap->ia_valid & ATTR_MODE))
 		iap->ia_mode = 0;
 	iap->ia_mode = (iap->ia_mode & S_IALLUGO) | type;
-- 
2.7.4