posix_acl: Clear SGID bit when setting file permissions

When file permissions are modified via chmod(2) and the user is not in
the owning group or capable of CAP_FSETID, the setgid bit is cleared in
inode_change_ok().  Setting a POSIX ACL via setxattr(2) sets the file
permissions as well as the new ACL, but doesn't clear the setgid bit in
a similar way; this allows to bypass the check in chmod(2).  Fix that.

References: CVE-2016-7097
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

1 files changed, 4 insertions, 8 deletions

diff --git a/fs/ext4/acl.c b/fs/ext4/acl.c
index c6601a476c02..dfa519979038 100644
--- a/fs/ext4/acl.c
+++ b/fs/ext4/acl.c
@@ -193,15 +193,11 @@ __ext4_set_acl(handle_t *handle, struct inode *inode, int type,
         case ACL_TYPE_ACCESS:
                 name_index = EXT4_XATTR_INDEX_POSIX_ACL_ACCESS;
                 if (acl) {
-                        error = posix_acl_equiv_mode(acl, &inode->i_mode);
-                        if (error < 0)
+                        error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+                        if (error)
                                 return error;
-                        else {
-                                inode->i_ctime = ext4_current_time(inode);
-                                ext4_mark_inode_dirty(handle, inode);
-                                if (error == 0)
-                                        acl = NULL;
-                        }
+                        inode->i_ctime = ext4_current_time(inode);
+                        ext4_mark_inode_dirty(handle, inode);
                 }
                 break;