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    SubjectRe: [RFC PATCH] f*xattr: allow O_PATH descriptors
    On Wed, Jun 8, 2022 at 2:57 PM Christian Brauner <brauner@kernel.org> wrote:
    >
    > On Tue, Jun 07, 2022 at 05:31:39PM +0200, Christian Göttsche wrote:
    > > From: Miklos Szeredi <mszeredi@redhat.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 [1] but requires a procfs,
    > > which might not be mounted e.g. inside of chroots, see[2].
    > >
    > > [1]: https://github.com/SELinuxProject/selinux/commit/7e979b56fd2cee28f647376a7233d2ac2d12ca50
    > > [2]: https://github.com/SELinuxProject/selinux/commit/de285252a1801397306032e070793889c9466845
    > >
    > > Original patch by Miklos Szeredi <mszeredi@redhat.com>
    > > https://patchwork.kernel.org/project/linux-fsdevel/patch/20200505095915.11275-6-mszeredi@redhat.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: linux-api@vger.kernel.org
    > > CC: linux-man@vger.kernel.org
    > > Signed-off-by: Christian Göttsche <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 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.

    >
    > 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.

    In effect, I think containing setxattr() can only be accomplished with LSM.

    Thanks,
    Amir.

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