linux-kernel - [PATCH v2] fs/{posix_acl,ext2,jfs,ceph}: apply umask if ACL support is disabled

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Message-Id: <20231009144340.418904-1-max.kellermann@ionos.com>
Date:   Mon,  9 Oct 2023 16:43:39 +0200
From:   Max Kellermann <max.kellermann@...os.com>
To:     Xiubo Li <xiubli@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>, Jan Kara <jack@...e.com>,
        Dave Kleikamp <shaggy@...nel.org>
Cc:     Max Kellermann <max.kellermann@...os.com>,
        ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ext4@...r.kernel.org, jfs-discussion@...ts.sourceforge.net
Subject: [PATCH v2] fs/{posix_acl,ext2,jfs,ceph}: apply umask if ACL support is disabled

One important implementation detail of the posix_acl_create() function
is that it applies the umask to the "mode" parameter.  If
CONFIG_FS_POSIX_ACL is disabled, this detail is missing and the umask
may not get applied.

This patch adds the missing code to posix_acl_create() and to three
filesystems that omit the posix_acl_create() call if their individual
ACL support is disabled (CONFIG_EXT2_FS_POSIX_ACL,
CONFIG_JFS_POSIX_ACL, CONFIG_CEPH_FS_POSIX_ACL).  If
posix_acl_create() never gets called, the umask needs to be applied
anyway.

This bug used to be exploitable easily with O_TMPFILE (see
https://bugzilla.kernel.org/show_bug.cgi?id=203625) but that part was
fixed by commit ac6800e279a2 ("fs: Add missing umask strip in
vfs_tmpfile") last year.  The bug may not be reachable by userspace
anymore, but since it is apparently still necessary to apply the umask
again in posix_acl_create(), there is no reason to assume it's not
necessary with ACL support is disabled.

Signed-off-by: Max Kellermann <max.kellermann@...os.com>
---
 fs/ceph/super.h           | 6 ++++++
 fs/ext2/acl.h             | 6 ++++++
 fs/jfs/jfs_acl.h          | 6 ++++++
 include/linux/posix_acl.h | 1 +
 4 files changed, 19 insertions(+)

diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index 51c7f2b14f6f..58349639bd57 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -1194,6 +1194,12 @@ static inline void ceph_forget_all_cached_acls(struct inode *inode)
 static inline int ceph_pre_init_acls(struct inode *dir, umode_t *mode,
 				     struct ceph_acl_sec_ctx *as_ctx)
 {
+	/* usually, the umask is applied by posix_acl_create(), but if
+	 * ACL support is disabled at compile time, we need to do it
+	 * here, because posix_acl_create() will never be called
+	 */
+	*mode &= ~current_umask();
+
 	return 0;
 }
 static inline void ceph_init_inode_acls(struct inode *inode,
diff --git a/fs/ext2/acl.h b/fs/ext2/acl.h
index 4a8443a2b8ec..0ecaa9c20c0c 100644
--- a/fs/ext2/acl.h
+++ b/fs/ext2/acl.h
@@ -67,6 +67,12 @@ extern int ext2_init_acl (struct inode *, struct inode *);
 
 static inline int ext2_init_acl (struct inode *inode, struct inode *dir)
 {
+	/* usually, the umask is applied by posix_acl_create(), but if
+	 * ACL support is disabled at compile time, we need to do it
+	 * here, because posix_acl_create() will never be called
+	 */
+	inode->i_mode &= ~current_umask();
+
 	return 0;
 }
 #endif
diff --git a/fs/jfs/jfs_acl.h b/fs/jfs/jfs_acl.h
index f892e54d0fcd..64a05e663a45 100644
--- a/fs/jfs/jfs_acl.h
+++ b/fs/jfs/jfs_acl.h
@@ -17,6 +17,12 @@ int jfs_init_acl(tid_t, struct inode *, struct inode *);
 static inline int jfs_init_acl(tid_t tid, struct inode *inode,
 			       struct inode *dir)
 {
+	/* usually, the umask is applied by posix_acl_create(), but if
+	 * ACL support is disabled at compile time, we need to do it
+	 * here, because posix_acl_create() will never be called
+	 */
+	inode->i_mode &= ~current_umask();
+
 	return 0;
 }
 
diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
index 0e65b3d634d9..54bc9b1061ca 100644
--- a/include/linux/posix_acl.h
+++ b/include/linux/posix_acl.h
@@ -128,6 +128,7 @@ static inline void cache_no_acl(struct inode *inode)
 static inline int posix_acl_create(struct inode *inode, umode_t *mode,
 		struct posix_acl **default_acl, struct posix_acl **acl)
 {
+	*mode &= ~current_umask();
 	*default_acl = *acl = NULL;
 	return 0;
 }
-- 
2.39.2