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
next prev 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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