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From: Aleksa Sarai <cyphar@cyphar.com>
To: mtk.manpages@gmail.com
Cc: linux-man@vger.kernel.org, linux-api@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	viro@zeniv.linux.org.uk, christian@brauner.io,
	Aleksa Sarai <cyphar@cyphar.com>
Subject: [PATCH] symlink.7: document magic-links more completely
Date: Wed, 10 Jun 2020 15:53:19 +1000	[thread overview]
Message-ID: <20200610055319.26374-1-cyphar@cyphar.com> (raw)
In-Reply-To: <20200414103524.wjhyfobzpjk236o7@yavin.dot.cyphar.com>

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@cyphar.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


  parent reply	other threads:[~2020-06-10  5:54 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-02 15:19 [PATCH man-pages v2 0/2] document openat2(2) Aleksa Sarai
2020-02-02 15:19 ` [PATCH man-pages v2 1/2] path_resolution.7: update to mention openat2(2) features Aleksa Sarai
2020-03-30 20:38   ` Michael Kerrisk (man-pages)
2020-02-02 15:19 ` [PATCH man-pages v2 2/2] openat2.2: document new openat2(2) syscall Aleksa Sarai
2020-03-30  9:08   ` Michael Kerrisk (man-pages)
2020-03-30  9:20     ` Aleksa Sarai
2020-03-30  9:36       ` Michael Kerrisk (man-pages)
2020-03-30  9:48         ` Aleksa Sarai
2020-03-30 20:43   ` Michael Kerrisk (man-pages)
2020-03-31 14:39     ` Aleksa Sarai
2020-04-01  6:38       ` Michael Kerrisk (man-pages)
2020-04-08 21:29         ` Michael Kerrisk (man-pages)
2020-04-12 16:49         ` Aleksa Sarai
2020-04-13  7:22           ` Michael Kerrisk (man-pages)
2020-04-14 10:35             ` Aleksa Sarai
2020-04-15 20:24               ` Michael Kerrisk (man-pages)
2020-05-04 10:17               ` Michael Kerrisk (man-pages)
2020-06-10  5:53               ` Aleksa Sarai [this message]
2020-06-19 13:00                 ` [PATCH] symlink.7: document magic-links more completely Michael Kerrisk (man-pages)

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