From mboxrd@z Thu Jan 1 00:00:00 1970 From: Aleksa Sarai Subject: Re: [PATCH RFC 1/3] symlink.7: document magic-links more completely Date: Wed, 9 Oct 2019 20:57:21 +1100 Message-ID: <20191009095721.ruheswshfxcgbbkp@yavin> References: <20191003145542.17490-1-cyphar@cyphar.com> <20191003145542.17490-2-cyphar@cyphar.com> <2fd9e82d-2a9c-cda9-0c17-3a20034eca1d@gmail.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="cllqcbo6bs3bqn24" Return-path: Content-Disposition: inline In-Reply-To: <2fd9e82d-2a9c-cda9-0c17-3a20034eca1d@gmail.com> Sender: linux-kernel-owner@vger.kernel.org To: "Michael Kerrisk (man-pages)" Cc: Al Viro , Christian Brauner , Aleksa Sarai , linux-man@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-api@vger.kernel.org --cllqcbo6bs3bqn24 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2019-10-09, Michael Kerrisk (man-pages) 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. > >=20 > > Signed-off-by: Aleksa Sarai >=20 > 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(-) > >=20 > > 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 >=20 > "magic link". As Jann points out, this is more normal English usage. >=20 > > +.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" o= f the > > +magic-link (unlike ordinary symlinks, magic-links have their own file = mode.) >=20 > 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 rul= es, > > 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 >=20 > "magic links" (and through the rest of the page). >=20 > > +can be found in certain pseudo-filesystems such as >=20 > pseudofilesystems >=20 > > +.BR proc (5) > > +(examples include > > +.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* .) > > +Unlike normal symlinks, magic-links are not resolved through >=20 > symbolic links >=20 > > +pathname-expansion, but instead act as direct references to the kernel= 's own >=20 > pathname expansion Will do all of the above. > > +representation of a file handle. As such, these magic-links allow user= s to > > +access files which cannot be referenced with normal paths (such as unl= inked > > +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 --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --cllqcbo6bs3bqn24 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQSxZm6dtfE8gxLLfYqdlLljIbnQEgUCXZ2u/gAKCRCdlLljIbnQ EjRSAQD3mdmhcqPvPn9QRxIs/5ozFNFVHki8T6u+CFbUm3DsCQD+LtXhgKHyrZ9s KEbVGN3G7nFnzi2ZDaPYj7apZfVNCgI= =THxq -----END PGP SIGNATURE----- --cllqcbo6bs3bqn24--