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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Fri, 19 Aug 2022 23:27:49 +0300
Message-ID: <CAOQ4uxhiyVixjDnDsMusfAPqP4DkbA0TfmOKGLa_L6T6s1JJjQ@mail.gmail.com>
Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors
To:     =?UTF-8?Q?Christian_G=C3=B6ttsche?= <cgzones@googlemail.com>
Cc:     Aleksa Sarai <cyphar@cyphar.com>,
        Christian Brauner <brauner@kernel.org>,
        SElinux list <selinux@vger.kernel.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Fri, Aug 19, 2022 at 9:05 PM Christian G=C3=B6ttsche
<cgzones@googlemail.com> wrote:
>
> On Wed, 22 Jun 2022 at 04:57, Aleksa Sarai <cyphar@cyphar.com> wrote:
> >
> > On 2022-06-20, Amir Goldstein <amir73il@gmail.com> wrote:
> > > To be a reasonable reaction to the currently broken API is
> > > to either accept the patch as is or request that setxattrat()
> > > will be added to provide the new functionality.
> >
> > Since the current functionality cannot be retroactively disabled as it
> > is being used already through /proc/self/fd/$n, adding
> > *xattrat(AT_EMPTY_PATH) doesn't really change what is currently possibl=
e
> > by userspace.
> >
> > I would say we should add *xattrat(2) and then we can add an upgrade
> > mask blocking it (and other operations) later.
> >
>
> It seems setxattrat() is the preferred way to continue.
> fsetxattr() would have one advantage though (w.r.t. SELinux):
>
> The steps to label a file are:
>   1. get the type of the file (via stat(2) family)
>   2. lookup the desired label from the label database via selabel_lookup(=
3)
>   3. assign the retrieved label to the file
>
> The label is sensitive to the file type, e.g.
>
>     $ matchpathcon -m file /etc/shadow
>     /etc/shadow     system_u:object_r:shadow_t:s0
>     $ matchpathcon -m lnk_file /etc/shadow
>     /etc/shadow     system_u:object_r:etc_t:s0
>
> Using the *at() family the file type could change between step 1. and 3.,
> which operating on an O_PATH file descriptor would prevent.

I don't understand the problem.

If you have an O_PATH fd, the object it represents does not change.
If you use fstatat(fd, ..., AT_EMPTY_PATH) (or fstat) and
setxattrat(fd, ..., AT_EMPTY_PATH), it prevents the race.

Thanks,
Amir.