On 2020-04-17, Michael Kerrisk (man-pages) <mtk.manpages@xxxxxxxxx> 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@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/ -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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