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

Jan Kara
2023-10-11T12:07:16Z

On Wed 11-10-23 12:51:12, Max Kellermann wrote:
> On Wed, Oct 11, 2023 at 12:05 PM Jan Kara <jack@suse.cz> wrote:
> > So I've checked some more and the kernel doc comments before
> > mode_strip_umask() and vfs_prepare_mode() make it pretty obvious - all
> > paths creating new inodes must be calling vfs_prepare_mode(). As a result
> > mode_strip_umask() which handles umask stripping for filesystems not
> > supporting posix ACLs. For filesystems that do support ACLs,
> > posix_acl_create() must be call and that handles umask stripping. So your
> > fix should not be needed. CCed some relevant people for confirmation.
>
> Thanks, Jan. Do you think the documentation is obvious enough, or
> shall I look around and try to improve the documentation? I'm not a FS
> expert, so it may be just my fault that it confused me.... I just
> analyzed the O_TMPFILE vulnerability four years ago (because it was
> reported to me as the maintainer of a userspace software).
>
> Apart from my doubts that this API contract is too error prone, I'm
> not quite sure if all filesystems really implement it properly.
>
> For example, overlayfs unconditionally sets SB_POSIXACL, even if the
> kernel has no ACL support. Would this ignore the umask? I'm not sure,
> overlayfs is a special beast.
> Then there's orangefs which allows setting the "acl" mount option (and
> thus SB_POSIXACL) even if the kernel has no ACL support. Same for gfs2
> and maybe cifs, maybe more, I didn't check them all.

Indeed, *that* looks like a bug. Good spotting! I'd say posix_acl_create()
defined in include/linux/posix_acl.h for the !CONFIG_FS_POSIX_ACL case
should be stripping mode using umask. Care to send a patch for this?

> The "mainstream" filesystems like ext4 seem to be implemented
> properly, though this is still too fragile for my taste... ext4 has
> the SB_POSIXACL code even if there's no kernel ACL support, but it is
> not reachable because EXT4_MOUNT_POSIX_ACL cannot be set from
> userspace in that case. The code looks suspicious, but is okay in the
> end - still not my taste.
>
> I see so much redundant code regarding the "acl" mount option in all
> filesystems. I believe the API should be designed in a way that it is
> safe-by-default, and shouldn't need very careful considerations in
> each and every filesystem, or else all filesystems repeat the same
> mistakes until the last one gets fixed.

So I definitely agree that we should handle as many things as possible in
VFS without relying on filesystems to get it right. Thus I agree VFS should
do the right thing even if the filesystem sets SB_POSIXACl when
!CONFIG_FS_POSIX_ACL.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR