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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Sat, 18 Jun 2022 12:11:08 +0300
Message-ID: <CAOQ4uxg6QLJ26pX8emXmUvq6jDDEH_Qq=Z4RPUK-jGLsZpHzfg@mail.gmail.com>
Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors
To:     Aleksa Sarai <cyphar@cyphar.com>,
        Christian Brauner <brauner@kernel.org>
Cc:     =?UTF-8?Q?Christian_G=C3=B6ttsche?= <cgzones@googlemail.com>,
        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 Sat, Jun 18, 2022 at 6:18 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
>
> On 2022-06-08, Amir Goldstein <amir73il@gmail.com> wrote:
> > On Wed, Jun 8, 2022 at 3:48 PM Christian Brauner <brauner@kernel.org> w=
rote:
> > >
> > > On Wed, Jun 08, 2022 at 03:28:52PM +0300, Amir Goldstein wrote:
> > > > On Wed, Jun 8, 2022 at 2:57 PM Christian Brauner <brauner@kernel.or=
g> wrote:
> > > > >
> > > > > On Tue, Jun 07, 2022 at 05:31:39PM +0200, Christian G=C3=B6ttsche=
 wrote:
> > > > > > From: Miklos Szeredi <mszeredi@redhat.com>
> > > > > >
> > > > > > Support file descriptors obtained via O_PATH for extended attri=
bute
> > > > > > operations.
> > > > > >
> > > > > > Extended attributes are for example used by SELinux for the sec=
urity
> > > > > > context of file objects. To avoid time-of-check-time-of-use iss=
ues while
> > > > > > setting those contexts it is advisable to pin the file in quest=
ion and
> > > > > > operate on a file descriptor instead of the path name. This can=
 be
> > > > > > emulated in userspace via /proc/self/fd/NN [1] but requires a p=
rocfs,
> > > > > > which might not be mounted e.g. inside of chroots, see[2].
> > > > > >
> > > > > > [1]: https://github.com/SELinuxProject/selinux/commit/7e979b56f=
d2cee28f647376a7233d2ac2d12ca50
> > > > > > [2]: https://github.com/SELinuxProject/selinux/commit/de285252a=
1801397306032e070793889c9466845
> > > > > >
> > > > > > Original patch by Miklos Szeredi <mszeredi@redhat.com>
> > > > > > https://patchwork.kernel.org/project/linux-fsdevel/patch/202005=
05095915.11275-6-mszeredi@redhat.com/
> > > > > >
> > > > > > > While this carries a minute risk of someone relying on the pr=
operty of
> > > > > > > xattr syscalls rejecting O_PATH descriptors, it saves the tro=
uble of
> > > > > > > introducing another set of syscalls.
> > > > > > >
> > > > > > > Only file->f_path and file->f_inode are accessed in these fun=
ctions.
> > > > > > >
> > > > > > > Current versions return EBADF, hence easy to detect the prese=
nse of
> > > > > > > this feature and fall back in case it's missing.
> > > > > >
> > > > > > CC: linux-api@vger.kernel.org
> > > > > > CC: linux-man@vger.kernel.org
> > > > > > Signed-off-by: Christian G=C3=B6ttsche <cgzones@googlemail.com>
> > > > > > ---
> > > > >
> > > > > I'd be somewhat fine with getxattr and listxattr but I'm worried =
that
> > > > > setxattr/removexattr waters down O_PATH semantics even more. I do=
n't
> > > > > want O_PATH fds to be useable for operations which are semantical=
ly
> > > > > equivalent to a write.
> > > >
> > > > It is not really semantically equivalent to a write if it works on =
a
> > > > O_RDONLY fd already.
> > >
> > > The fact that it works on a O_RDONLY fd has always been weird. And is
> > > probably a bug. If you look at xattr_permission() you can see that it
> >
> > Bug or no bug, this is the UAPI. It is not fixable anymore.
> >
> > > checks for MAY_WRITE for set operations... setxattr() writes to disk =
for
> > > real filesystems. I don't know how much closer to a write this can ge=
t.
> > >
> > > In general, one semantic aberration doesn't justify piling another on=
e
> > > on top.
> > >
> > > (And one thing that speaks for O_RDONLY is at least that it actually
> > > opens the file wheres O_PATH doesn't.)
> >
> > Ok. I care mostly about consistent UAPI, so if you want to set the
> > rule that modify f*() operations are not allowed to use O_PATH fd,
> > I can live with that, although fcntl(2) may be breaking that rule, but
> > fine by me.
> > It's good to have consistent rules and it's good to add a new UAPI for
> > new behavior.
> >
> > However...
> >
> > >
> > > >
> > > > >
> > > > > In sensitive environments such as service management/container ru=
ntimes
> > > > > we often send O_PATH fds around precisely because it is restricte=
d what
> > > > > they can be used for. I'd prefer to not to plug at this string.
> > > >
> > > > But unless I am mistaken, path_setxattr() and syscall_fsetxattr()
> > > > are almost identical w.r.t permission checks and everything else.
> > > >
> > > > So this change introduces nothing new that a user in said environme=
nt
> > > > cannot already accomplish with setxattr().
> > > >
> > > > Besides, as the commit message said, doing setxattr() on an O_PATH
> > > > fd is already possible with setxattr("/proc/self/$fd"), so whatever=
 security
> > > > hole you are trying to prevent is already wide open.
> > >
> > > That is very much a something that we're trying to restrict for this
> > > exact reason and is one of the main motivator for upgrade mask in
> > > openat2(). If I want to send a O_PATH around I want it to not be
> > > upgradable. Aleksa is working on upgrade masks with openat2() (see [1=
]
> > > and part of the original patchset in [2]. O_PATH semantics don't need=
 to
> > > become weird.
> > >
> > > [1]: https://lore.kernel.org/all/20220526130355.fo6gzbst455fxywy@senk=
u
> > > [2]: https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20190728=
010207.9781-8-cyphar@cyphar.com
> >
> > ... thinking forward, if this patch is going to be rejected, the patch =
that
> > will follow is *xattrat() syscalls.
> >
> > What will you be able to argue then?
> >
> > There are several *at() syscalls that modify metadata.
> > fchownat(.., AT_EMPTY_PATH) is intentionally designed for this.
> >
> > Do you intend to try and block setxattrat()?
> > Just try and block setxattrat(.., AT_EMPTY_PATH)?
> > those *at() syscalls have real use cases to avoid TOCTOU races.
> > Do you propose that applications will have to use fsetxattr() on an ope=
n
> > file to avert races?
> >
> > I completely understand the idea behind upgrade masks
> > for limiting f_mode, but I don't know if trying to retroactively
> > change semantics of setxattr() in the move to setxattrat()
> > is going to be a good idea.
>
> The goal would be that the semantics of fooat(<fd>, AT_EMPTY_PATH) and
> foo(/proc/self/fd/<fd>) should always be identical, and the current
> semantics of /proc/self/fd/<fd> are too leaky so we shouldn't always
> assume that keeping them makes sense (the most obvious example is being
> able to do tricks to open /proc/$pid/exe as O_RDWR).

Please make a note that I have applications relying on current magic symlin=
k
semantics w.r.t setxattr() and other metadata operations, and the libselinu=
x
commit linked from the patch commit message proves that magic symlink
semantics are used in the wild, so it is not likely that those semantics co=
uld
be changed, unless userspace breakage could be justified by fixing a seriou=
s
security issue (i.e. open /proc/$pid/exe as O_RDWR).

>
> I suspect that the long-term solution would be to have more upgrade
> masks so that userspace can opt-in to not allowing any kind of
> (metadata) write access through a particular file descriptor. You're
> quite right that we have several metadata write AT_EMPTY_PATH APIs, and
> so we can't retroactively block /everything/ but we should try to come
> up with less leaky rules by default if it won't break userspace.
>

Ok, let me try to say this in my own words using an example to see that
we are all on the same page:

- lsetxattr(PATH_TO_FILE,..) has inherent TOCTOU races
- fsetxattr(fd,...) is not applicable for symbolic links
- setxattr("/proc/self/fd/<fd>",...) is the current API to avoid TOCTOU rac=
es
  when setting xattr on symbolic links
- setxattrat(o_path_fd, "", ..., AT_EMPTY_PATH) is proposed as a the
  "new API" for setting xattr on symlinks (and special files)
- The new API is going to be more strict than the old magic symlink API
- *If* it turns out to not break user applications, old API can also become
  more strict to align with new API (unlikely the case for setxattr())
- This will allow sandboxed containers to opt-out of the "old API", by
  restricting access to /proc/self/fd and to implement more fine grained
  control over which metadata operations are allowed on an O_PATH fd

Did I understand the plan correctly?
Do you agree with me that the plan to keep AT_EMPTY_PATH and
magic symlink semantics may not be realistic?

Thanks,
Amir.