linux-kernel - [PATCH 4.4 135/168] ovl: fix uid/gid when creating over whiteout

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Date:   Wed, 24 Apr 2019 19:09:39 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Stephen Smalley <sds@...ho.nsa.gov>,
        Miklos Szeredi <mszeredi@...hat.com>,
        "Sasha Levin (Microsoft)" <sashal@...nel.org>
Subject: [PATCH 4.4 135/168] ovl: fix uid/gid when creating over whiteout

[ Upstream commit d0e13f5bbe4be7c8f27736fc40503dcec04b7de0 ]

Fix a regression when creating a file over a whiteout.  The new
file/directory needs to use the current fsuid/fsgid, not the ones from the
mounter's credentials.

The refcounting is a bit tricky: prepare_creds() sets an original refcount,
override_creds() gets one more, which revert_cred() drops.  So

  1) we need to expicitly put the mounter's credentials when overriding
     with the updated one

  2) we need to put the original ref to the updated creds (and this can
     safely be done before revert_creds(), since we'll still have the ref
     from override_creds()).

Reported-by: Stephen Smalley <sds@...ho.nsa.gov>
Fixes: 3fe6e52f0626 ("ovl: override creds with the ones from the superblock mounter")
Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
Signed-off-by: Sasha Levin (Microsoft) <sashal@...nel.org>
---
 fs/overlayfs/dir.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index f8aa54272121..eedacae889b9 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -408,12 +408,21 @@ static int ovl_create_or_link(struct dentry *dentry, int mode, dev_t rdev,
 		err = ovl_create_upper(dentry, inode, &stat, link, hardlink);
 	} else {
 		const struct cred *old_cred;
+		struct cred *override_cred;
 
 		old_cred = ovl_override_creds(dentry->d_sb);
 
-		err = ovl_create_over_whiteout(dentry, inode, &stat, link,
-					       hardlink);
+		err = -ENOMEM;
+		override_cred = prepare_creds();
+		if (override_cred) {
+			override_cred->fsuid = old_cred->fsuid;
+			override_cred->fsgid = old_cred->fsgid;
+			put_cred(override_creds(override_cred));
+			put_cred(override_cred);
 
+			err = ovl_create_over_whiteout(dentry, inode, &stat,
+						       link, hardlink);
+		}
 		revert_creds(old_cred);
 	}
 
-- 
2.19.1