From: Aleksa Sarai <cyphar@cyphar.com>
To: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
Christian Brauner <christian@brauner.io>,
Aleksa Sarai <asarai@suse.de>,
linux-man@vger.kernel.org, linux-api@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 1/3] symlink.7: document magic-links more completely
Date: Wed, 9 Oct 2019 20:57:21 +1100 [thread overview]
Message-ID: <20191009095721.ruheswshfxcgbbkp@yavin> (raw)
In-Reply-To: <2fd9e82d-2a9c-cda9-0c17-3a20034eca1d@gmail.com>
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On 2019-10-09, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
> On 10/3/19 4:55 PM, Aleksa Sarai 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>
>
> Thanks for doing this. Some comments below.
No problem -- just a heads-up that I'm going to split off the magic-link
changes from the openat2(2) series (there are quite a few things that
need to be done). So I will drop this man page for now.
> > ---
> > 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
>
> "magic link". As Jann points out, this is more normal English usage.
>
> > +.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.)
>
> Remove the hyphens (magic link). And also, as someone else pointed out,
> manual pages fairly consistently uses the term "symbolic link"
> (written in full).
Will do.
> You use the term "file mode" here. Do you mean the file permissions bits?
Yes.
> If yes, it is a bit misleading to suggest that symbolic links don't
> have these mode bits. They do, but--as noted in the existing symlink(7)
> manual page text--these bits are ignored. I suggest just removing the
> parenthesized text.
I was trying to say that their file mode can be non-0777 -- but I can
just drop the entire thing.
> > +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
>
> "magic links" (and through the rest of the page).
>
> > +can be found in certain pseudo-filesystems such as
>
> pseudofilesystems
>
> > +.BR proc (5)
> > +(examples include
> > +.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* .)
> > +Unlike normal symlinks, magic-links are not resolved through
>
> symbolic links
>
> > +pathname-expansion, but instead act as direct references to the kernel's own
>
> pathname expansion
Will do all of the above.
> > +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
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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next prev parent reply other threads:[~2019-10-09 10:06 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-03 14:55 [PATCH RFC 0/3] document openat2(2) patch series Aleksa Sarai
2019-10-03 14:55 ` [PATCH RFC 1/3] symlink.7: document magic-links more completely Aleksa Sarai
2019-10-07 16:36 ` Jann Horn
2019-10-08 1:33 ` Aleksa Sarai
2019-10-09 7:55 ` Michael Kerrisk (man-pages)
2019-10-09 9:57 ` Aleksa Sarai [this message]
2020-04-17 11:39 ` Michael Kerrisk (man-pages)
2020-04-17 15:37 ` Aleksa Sarai
2020-05-15 11:42 ` Michael Kerrisk (man-pages)
2019-10-03 14:55 ` [PATCH RFC 2/3] open.2: add O_EMPTYPATH documentation Aleksa Sarai
2019-10-09 8:01 ` Michael Kerrisk (man-pages)
2019-10-09 10:00 ` Aleksa Sarai
2019-10-03 14:55 ` [PATCH RFC 3/3] openat2.2: document new openat2(2) syscall Aleksa Sarai
2019-10-09 8:36 ` Michael Kerrisk (man-pages)
2019-10-09 10:17 ` Aleksa Sarai
2019-10-09 10:32 ` Michael Kerrisk (man-pages)
2019-10-03 14:55 ` [PATCH RFC 3/3] openat2.2: document new syscall Aleksa Sarai
2019-10-03 15:00 ` Aleksa Sarai
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