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

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2020-04-17, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
> 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?

Yes, this is the patch I was thinking of -- but since the whole "magic
link mode" semantics weren't in the openat2() series that was merged,
this would need a refresh. Also I feel that magic-links probably deserve
a slightly longer explanation than I gave here.

> 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/


-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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