linux-kernel - [PATCH] symlink.7: document magic-links more completely

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Message-Id: <20200610055319.26374-1-cyphar@cyphar.com>
Date:   Wed, 10 Jun 2020 15:53:19 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     mtk.manpages@...il.com
Cc:     linux-man@...r.kernel.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk, christian@...uner.io,
        Aleksa Sarai <cyphar@...har.com>
Subject: [PATCH] symlink.7: document magic-links more completely

Hi Michael,

Sorry for the delay and here is the patch I promised in this thread.

--8<---------------------------------------------------------------------8<--

Traditionally, magic-links have not been a well-understood topic in
Linux. This helps clarify some of the terminology used in openat2.2.

Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
 man7/symlink.7 | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/man7/symlink.7 b/man7/symlink.7
index 07b1db3a3764..ed99bc4236f1 100644
--- a/man7/symlink.7
+++ b/man7/symlink.7
@@ -84,6 +84,21 @@ 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.
 .SS Symbolic link ownership, permissions, and timestamps
 The owner and group of an existing symbolic link can be changed
 using
@@ -99,16 +114,14 @@ 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,
+though this mode is not currently used in any permission checks.
+
 .\"
 .\" The
 .\" 4.4BSD
-- 
2.26.2