From: Amir Goldstein <email@example.com>
To: Christian Brauner <firstname.lastname@example.org>
Cc: "Christian Göttsche" <email@example.com>,
firstname.lastname@example.org, "Miklos Szeredi" <email@example.com>,
"Linux API" <firstname.lastname@example.org>,
"Alexander Viro" <email@example.com>,
"Aleksa Sarai" <firstname.lastname@example.org>
Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors
Date: Wed, 8 Jun 2022 18:12:29 +0300 [thread overview]
Message-ID: <CAOQ4uxjP7kC95ou56wabVhQcc2vkNcD-8usYhLhbLOoJZ-jkOw@mail.gmail.com> (raw)
On Wed, Jun 8, 2022 at 3:48 PM Christian Brauner <email@example.com> wrote:
> On Wed, Jun 08, 2022 at 03:28:52PM +0300, Amir Goldstein wrote:
> > On Wed, Jun 8, 2022 at 2:57 PM Christian Brauner <firstname.lastname@example.org> wrote:
> > >
> > > On Tue, Jun 07, 2022 at 05:31:39PM +0200, Christian Göttsche wrote:
> > > > From: Miklos Szeredi <email@example.com>
> > > >
> > > > Support file descriptors obtained via O_PATH for extended attribute
> > > > operations.
> > > >
> > > > Extended attributes are for example used by SELinux for the security
> > > > context of file objects. To avoid time-of-check-time-of-use issues while
> > > > setting those contexts it is advisable to pin the file in question and
> > > > operate on a file descriptor instead of the path name. This can be
> > > > emulated in userspace via /proc/self/fd/NN  but requires a procfs,
> > > > which might not be mounted e.g. inside of chroots, see.
> > > >
> > > > : https://github.com/SELinuxProject/selinux/commit/7e979b56fd2cee28f647376a7233d2ac2d12ca50
> > > > : https://github.com/SELinuxProject/selinux/commit/de285252a1801397306032e070793889c9466845
> > > >
> > > > Original patch by Miklos Szeredi <firstname.lastname@example.org>
> > > > https://email@example.com/
> > > >
> > > > > While this carries a minute risk of someone relying on the property of
> > > > > xattr syscalls rejecting O_PATH descriptors, it saves the trouble of
> > > > > introducing another set of syscalls.
> > > > >
> > > > > Only file->f_path and file->f_inode are accessed in these functions.
> > > > >
> > > > > Current versions return EBADF, hence easy to detect the presense of
> > > > > this feature and fall back in case it's missing.
> > > >
> > > > CC: firstname.lastname@example.org
> > > > CC: email@example.com
> > > > Signed-off-by: Christian Göttsche <firstname.lastname@example.org>
> > > > ---
> > >
> > > I'd be somewhat fine with getxattr and listxattr but I'm worried that
> > > setxattr/removexattr waters down O_PATH semantics even more. I don't
> > > want O_PATH fds to be useable for operations which are semantically
> > > 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 get.
> In general, one semantic aberration doesn't justify piling another one
> 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
> > >
> > > In sensitive environments such as service management/container runtimes
> > > we often send O_PATH fds around precisely because it is restricted 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 environment
> > 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 
> and part of the original patchset in . O_PATH semantics don't need to
> become weird.
> : https://lore.kernel.org/all/20220526130355.fo6gzbst455fxywy@senku
> : https://email@example.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 open
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.
And forgive me if I am failing to see the big picture.
It is certainly a possibility.
next prev parent reply other threads:[~2022-06-08 15:17 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-07 15:31 [RFC PATCH] f*xattr: allow O_PATH descriptors Christian Göttsche
2022-06-08 5:13 ` Amir Goldstein
2022-06-08 11:27 ` Christian Brauner
2022-06-08 12:28 ` Amir Goldstein
2022-06-08 12:48 ` Christian Brauner
2022-06-08 15:12 ` Amir Goldstein [this message]
2022-06-09 8:56 ` Christian Brauner
2022-06-18 3:18 ` Aleksa Sarai
2022-06-18 9:11 ` Amir Goldstein
2022-06-18 11:19 ` Christian Göttsche
2022-06-18 15:30 ` Amir Goldstein
2022-06-20 6:07 ` Aleksa Sarai
2022-06-20 7:45 ` Amir Goldstein
2022-06-22 2:57 ` Aleksa Sarai
2022-08-19 18:05 ` Christian Göttsche
2022-08-19 20:27 ` Amir Goldstein
2022-06-08 16:53 ` Andreas Dilger
2022-06-09 4:35 ` Amir Goldstein
2022-06-09 9:14 ` Christian Göttsche
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