Introduce is_owner_or_cap() to wrap CAP_FOWNER use with fsuid check

Introduce is_owner_or_cap() macro in fs.h, and convert over relevant
users to it. This is done because we want to avoid bugs in the future
where we check for only effective fsuid of the current task against a
file's owning uid, without simultaneously checking for CAP_FOWNER as
well, thus violating its semantics.
[ XFS uses special macros and structures, and in general looked ...
untouchable, so we leave it alone -- but it has been looked over. ]

The (current->fsuid != inode->i_uid) check in generic_permission() and
exec_permission_lite() is left alone, because those operations are
covered by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH. Similarly operations
falling under the purview of CAP_CHOWN and CAP_LEASE are also left alone.

Signed-off-by: Satyam Sharma <ssatyam@cse.iitk.ac.in>
Cc: Al Viro <viro@ftp.linux.org.uk>
Acked-by: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 4 insertions, 4 deletions

diff --git a/fs/ext4/acl.c b/fs/ext4/acl.c
index 9e882546d91a..a8bae8cd1d5d 100644
--- a/fs/ext4/acl.c
+++ b/fs/ext4/acl.c
@@ -489,7 +489,7 @@ ext4_xattr_set_acl(struct inode *inode, int type, const void *value,
 
         if (!test_opt(inode->i_sb, POSIX_ACL))
                 return -EOPNOTSUPP;
-        if ((current->fsuid != inode->i_uid) && !capable(CAP_FOWNER))
+        if (!is_owner_or_cap(inode))
                 return -EPERM;
 
         if (value) {
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 500567dd53b6..7b4aa4543c83 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -40,7 +40,7 @@ int ext4_ioctl (struct inode * inode, struct file * filp, unsigned int cmd,
                 if (IS_RDONLY(inode))
                         return -EROFS;
 
-                if ((current->fsuid != inode->i_uid) && !capable(CAP_FOWNER))
+                if (!is_owner_or_cap(inode))
                         return -EACCES;
 
                 if (get_user(flags, (int __user *) arg))
@@ -121,7 +121,7 @@ flags_err:
                 __u32 generation;
                 int err;
 
-                if ((current->fsuid != inode->i_uid) && !capable(CAP_FOWNER))
+                if (!is_owner_or_cap(inode))
                         return -EPERM;
                 if (IS_RDONLY(inode))
                         return -EROFS;
@@ -180,7 +180,7 @@ flags_err:
                 if (IS_RDONLY(inode))
                         return -EROFS;
 
-                if ((current->fsuid != inode->i_uid) && !capable(CAP_FOWNER))
+                if (!is_owner_or_cap(inode))
                         return -EACCES;
 
                 if (get_user(rsv_window_size, (int __user *)arg))