[Lucid SRU PATCH] eCryptfs: Copy up POSIX ACL and read-only flags from lower mount

[Tim Gardner]

From: Tyler Hicks <tyhicks at canonical.com>

When the eCryptfs mount options do not include '-o acl', but the lower
filesystem's mount options do include 'acl', the MS_POSIXACL flag is not
flipped on in the eCryptfs super block flags. This flag is what the VFS
checks in do_last() when deciding if the current umask should be applied
to a newly created inode's mode or not. When a default POSIX ACL mask is
set on a directory, the current umask is incorrectly applied to new
inodes created in the directory. This patch ignores the MS_POSIXACL flag
passed into ecryptfs_mount() and sets the flag on the eCryptfs super
block depending on the flag's presence on the lower super block.

Additionally, it is incorrect to allow a writeable eCryptfs mount on top
of a read-only lower mount. This missing check did not allow writes to
the read-only lower mount because permissions checks are still performed
on the lower filesystem's objects but it is best to simply not allow a
rw mount on top of ro mount. However, a ro eCryptfs mount on top of a rw
mount is valid and still allowed.

https://launchpad.net/bugs/1009207

Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
Reported-by: Stefan Beller <stefanbeller at googlemail.com>
Cc: John Johansen <john.johansen at canonical.com>
(back ported from commit 069ddcda37b2cf5bb4b6031a944c0e9359213262)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 fs/ecryptfs/main.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/ecryptfs/main.c b/fs/ecryptfs/main.c
index e2f18ad..783f7fc 100644
--- a/fs/ecryptfs/main.c
+++ b/fs/ecryptfs/main.c
@@ -589,6 +589,15 @@ static int ecryptfs_read_super(struct super_block *sb, const char *dev_name,
 	}
 
 	ecryptfs_set_superblock_lower(sb, path.dentry->d_sb);
+
+	/**
+	 * Set the POSIX ACL flag based on whether they're enabled in the lower
+	 * mount. Force a read-only eCryptfs mount if the lower mount is ro.
+	 * Allow a ro eCryptfs mount even when the lower mount is rw.
+	 */
+	sb->s_flags &= ~MS_POSIXACL;
+	sb->s_flags |= path.dentry->d_sb->s_flags & (MS_RDONLY | MS_POSIXACL);
+
 	sb->s_maxbytes = path.dentry->d_sb->s_maxbytes;
 	sb->s_blocksize = path.dentry->d_sb->s_blocksize;
 	ecryptfs_set_dentry_lower(sb->s_root, path.dentry);
-- 
1.7.9.5