This is a kernel rather than a setfacl bug. The bug was introduced in v4.9-rc1 by commit 07393101, and fixed around v4.13-rc4 by various commits depending on the filesystem used. The ext4 fix is a3bb2d55. Here are some details:
commit 073931017b49d9458aa351605b43a7e34598caef Author: Jan Kara <[email protected]> Date: Mon Sep 19 17:39:09 2016 +0200 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 <[email protected]> Reviewed-by: Jeff Layton <[email protected]> Signed-off-by: Jan Kara <[email protected]> Signed-off-by: Andreas Gruenbacher <[email protected]> commit a3bb2d5587521eea6dab2d05326abb0afb460abd Author: Jan Kara <[email protected]> Date: Sun Jul 30 23:33:01 2017 -0400 ext4: Don't clear SGID when inheriting ACLs When new directory 'DIR1' is created in a directory 'DIR0' with SGID bit set, DIR1 is expected to have SGID bit set (and owning group equal to the owning group of 'DIR0'). However when 'DIR0' also has some default ACLs that 'DIR1' inherits, setting these ACLs will result in SGID bit on 'DIR1' to get cleared if user is not member of the owning group. Fix the problem by moving posix_acl_update_mode() out of __ext4_set_acl() into ext4_set_acl(). That way the function will not be called when inheriting ACLs which is what we want as it prevents SGID bit clearing and the mode has been properly set by posix_acl_create() anyway. Fixes: 073931017b49d9458aa351605b43a7e34598caef CC: [email protected] Signed-off-by: Theodore Ts'o <[email protected]> Signed-off-by: Jan Kara <[email protected]> Reviewed-by: Andreas Gruenbacher <[email protected]>
