Re: [PATCH RFC 1/3] symlink.7: document magic-links more completely

Michael Kerrisk (man-pages)
2020-04-17T11:39:55Z

Hi Aleksa,

Re our discussion of documentation to be added for magic symlinks,
there was the patch below, which got paused. I guess this just needs a
light refresh?

Thanks,

Michael

On Thu, 3 Oct 2019 at 16:56, Aleksa Sarai <cyphar@cyphar.com> wrote:
>
> Traditionally, magic-links have not been a well-understood topic in
> Linux. Given the new changes in their semantics (related to the link
> mode of trailing magic-links), it seems like a good opportunity to shine
> more light on magic-links and their semantics.
>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
> man7/path_resolution.7 | 15 +++++++++++++++
> man7/symlink.7 | 39 ++++++++++++++++++++++++++++++---------
> 2 files changed, 45 insertions(+), 9 deletions(-)
>
> diff --git a/man7/path_resolution.7 b/man7/path_resolution.7
> index 07664ed8faec..46f25ec4cdfa 100644
> --- a/man7/path_resolution.7
> +++ b/man7/path_resolution.7
> @@ -136,6 +136,21 @@ we are just creating it.
> The details on the treatment
> of the final entry are described in the manual pages of the specific
> system calls.
> +.PP
> +Since Linux 5.FOO, if the final entry is a "magic-link" (see
> +.BR symlink (7)),
> +and the user is attempting to
> +.BR open (2)
> +it, then there is an additional permission-related restriction applied to the
> +operation: the requested access mode must not exceed the "link mode" of the
> +magic-link (unlike ordinary symlinks, magic-links have their own file mode.)
> +For example, if
> +.I /proc/[pid]/fd/[num]
> +has a link mode of
> +.BR 0500 ,
> +unprivileged users are not permitted to
> +.BR open ()
> +the magic-link for writing.
> .SS . and ..
> By convention, every directory has the entries "." and "..",
> which refer to the directory itself and to its parent directory,
> diff --git a/man7/symlink.7 b/man7/symlink.7
> index 9f5bddd5dc21..33f0ec703acd 100644
> --- a/man7/symlink.7
> +++ b/man7/symlink.7
> @@ -84,6 +84,25 @@ as they are implemented on Linux and other systems,
> are outlined here.
> It is important that site-local applications also conform to these rules,
> so that the user interface can be as consistent as possible.
> +.SS Magic-links
> +There is a special class of symlink-like objects known as "magic-links" which
> +can be found in certain pseudo-filesystems such as
> +.BR proc (5)
> +(examples include
> +.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* .)
> +Unlike normal symlinks, magic-links are not resolved through
> +pathname-expansion, but instead act as direct references to the kernel's own
> +representation of a file handle. As such, these magic-links allow users to
> +access files which cannot be referenced with normal paths (such as unlinked
> +files still referenced by a running program.)
> +.PP
> +Because they can bypass ordinary
> +.BR mount_namespaces (7)-based
> +restrictions, magic-links have been used as attack vectors in various exploits.
> +As such (since Linux 5.FOO), there are additional restrictions placed on the
> +re-opening of magic-links (see
> +.BR path_resolution (7)
> +for more details.)
> .SS Symbolic link ownership, permissions, and timestamps
> The owner and group of an existing symbolic link can be changed
> using
> @@ -99,16 +118,18 @@ of a symbolic link can be changed using
> or
> .BR lutimes (3).
> .PP
> -On Linux, the permissions of a symbolic link are not used
> -in any operations; the permissions are always
> -0777 (read, write, and execute for all user categories),
> .\" Linux does not currently implement an lchmod(2).
> -and can't be changed.
> -(Note that there are some "magic" symbolic links in the
> -.I /proc
> -directory tree\(emfor example, the
> -.IR /proc/[pid]/fd/*
> -files\(emthat have different permissions.)
> +On Linux, the permissions of an ordinary symbolic link are not used in any
> +operations; the permissions are always 0777 (read, write, and execute for all
> +user categories), and can't be changed.
> +.PP
> +However, magic-links do not follow this rule. They can have a non-0777 mode,
> +which is used for permission checks when the final
> +component of an
> +.BR open (2)'s
> +path is a magic-link (see
> +.BR path_resolution (7).)
> +
> .\"
> .\" The
> .\" 4.4BSD
> --
> 2.23.0
>

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/