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@xxxxxxxxxx> 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@xxxxxxxxxx> > --- > 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/
