From: Michal Hocko <mhocko@kernel.org>
To: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: linux-api@vger.kernel.org, Khalid Aziz <khalid.aziz@oracle.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Andrew Morton <akpm@linux-foundation.org>,
Russell King - ARM Linux <linux@armlinux.org.uk>,
Andrea Arcangeli <aarcange@redhat.com>,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
linux-arch@vger.kernel.org, Florian Weimer <fweimer@redhat.com>,
John Hubbard <jhubbard@nvidia.com>,
Matthew Wilcox <willy@infradead.org>,
Jann Horn <jannh@google.com>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
Cyril Hrubis <chrubis@suse.cz>, Pavel Machek <pavel@ucw.cz>,
Michal Hocko <mhocko@suse.com>
Subject: [PATCH 2/2] mmap.2: MAP_FIXED updated documentation
Date: Wed, 13 Dec 2017 10:31:10 +0100 [thread overview]
Message-ID: <20171213093110.3550-2-mhocko@kernel.org> (raw)
In-Reply-To: <20171213093110.3550-1-mhocko@kernel.org>
From: John Hubbard <jhubbard@nvidia.com>
-- Expand the documentation to discuss the hazards in
enough detail to allow avoiding them.
-- Mention the upcoming MAP_FIXED_SAFE flag.
-- Enhance the alignment requirement slightly.
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Jann Horn <jannh@google.com>
CC: Matthew Wilcox <willy@infradead.org>
CC: Michal Hocko <mhocko@kernel.org>
CC: Mike Rapoport <rppt@linux.vnet.ibm.com>
CC: Cyril Hrubis <chrubis@suse.cz>
CC: Pavel Machek <pavel@ucw.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
man2/mmap.2 | 32 ++++++++++++++++++++++++++++++--
1 file changed, 30 insertions(+), 2 deletions(-)
diff --git a/man2/mmap.2 b/man2/mmap.2
index 02d391697ce6..cb8789daec2d 100644
--- a/man2/mmap.2
+++ b/man2/mmap.2
@@ -212,8 +212,9 @@ Don't interpret
.I addr
as a hint: place the mapping at exactly that address.
.I addr
-must be a multiple of the page size.
-If the memory region specified by
+must be suitably aligned: for most architectures a multiple of page
+size is sufficient; however, some architectures may impose additional
+restrictions. If the memory region specified by
.I addr
and
.I len
@@ -226,6 +227,33 @@ Software that aspires to be portable should use this option with care, keeping
in mind that the exact layout of a process' memory map is allowed to change
significantly between kernel versions, C library versions, and operating system
releases.
+.IP
+Furthermore, this option is extremely hazardous (when used on its own), because
+it forcibly removes pre-existing mappings, making it easy for a multi-threaded
+process to corrupt its own address space.
+.IP
+For example, thread A looks through
+.I /proc/<pid>/maps
+and locates an available
+address range, while thread B simultaneously acquires part or all of that same
+address range. Thread A then calls mmap(MAP_FIXED), effectively overwriting
+the mapping that thread B created.
+.IP
+Thread B need not create a mapping directly; simply making a library call
+that, internally, uses
+.I dlopen(3)
+to load some other shared library, will
+suffice. The dlopen(3) call will map the library into the process's address
+space. Furthermore, almost any library call may be implemented using this
+technique.
+Examples include brk(2), malloc(3), pthread_create(3), and the PAM libraries
+(http://www.linux-pam.org).
+.IP
+Newer kernels
+(Linux 4.16 and later) have a
+.B MAP_FIXED_SAFE
+option that avoids the corruption problem; if available, MAP_FIXED_SAFE
+should be preferred over MAP_FIXED.
.TP
.BR MAP_FIXED_SAFE " (since Linux 4.16)"
Similar to MAP_FIXED with respect to the
--
2.15.0
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next prev parent reply other threads:[~2017-12-13 9:31 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-13 9:25 [PATCH v2 0/2] mm: introduce MAP_FIXED_SAFE Michal Hocko
2017-12-13 9:25 ` [PATCH 1/2] " Michal Hocko
2017-12-13 12:50 ` Matthew Wilcox
2017-12-13 13:01 ` Michal Hocko
2017-12-13 9:25 ` [PATCH 2/2] fs, elf: drop MAP_FIXED usage from elf_map Michal Hocko
2017-12-16 0:49 ` [2/2] " Andrei Vagin
2017-12-18 9:13 ` Michal Hocko
2017-12-18 18:12 ` Andrei Vagin
2017-12-18 18:49 ` [PATCH] mm: don't use the same value for MAP_FIXED_SAFE and MAP_SYNC Andrei Vagin
2017-12-18 20:41 ` Michal Hocko
2018-04-18 10:51 ` [PATCH 2/2] fs, elf: drop MAP_FIXED usage from elf_map Tetsuo Handa
2018-04-18 11:33 ` Michal Hocko
2018-04-18 11:43 ` Tetsuo Handa
2018-04-18 11:55 ` Michal Hocko
2018-04-18 14:07 ` [PATCH v2] fs, elf: don't complain MAP_FIXED_NOREPLACE unless -EEXIST error Tetsuo Handa
2018-04-19 5:57 ` Michal Hocko
2017-12-13 9:31 ` [PATCH 1/2] mmap.2: document new MAP_FIXED_SAFE flag Michal Hocko
2017-12-13 9:31 ` Michal Hocko [this message]
2017-12-13 12:55 ` [PATCH 2/2] mmap.2: MAP_FIXED updated documentation Pavel Machek
2017-12-13 13:03 ` Cyril Hrubis
2017-12-13 13:04 ` Michal Hocko
2017-12-13 13:09 ` Pavel Machek
2017-12-13 13:16 ` Michal Hocko
2017-12-13 13:21 ` Pavel Machek
2017-12-13 13:35 ` Michal Hocko
2017-12-13 14:40 ` Cyril Hrubis
2017-12-13 23:19 ` Kees Cook
2017-12-14 7:07 ` Michal Hocko
2017-12-18 19:12 ` Michael Kerrisk (man-pages)
2017-12-18 20:19 ` Kees Cook
2017-12-18 20:33 ` Matthew Wilcox
2017-12-21 12:38 ` Michael Ellerman
2017-12-21 14:59 ` known bad patch in -mm tree was " Pavel Machek
2017-12-21 15:08 ` Michal Hocko
2017-12-21 22:24 ` Andrew Morton
2017-12-22 0:06 ` Michael Ellerman
2017-12-14 2:52 ` Jann Horn
2017-12-14 5:28 ` John Hubbard
2017-12-14 23:06 ` John Hubbard
2017-12-14 23:10 ` Jann Horn
2017-12-13 12:25 ` [PATCH v2 0/2] mm: introduce MAP_FIXED_SAFE Matthew Wilcox
2017-12-13 12:34 ` Michal Hocko
2017-12-13 17:13 ` Kees Cook
2017-12-15 9:02 ` Michael Ellerman
2017-12-14 0:32 ` Andrew Morton
2017-12-14 1:35 ` David Goldblatt
2017-12-14 1:42 ` David Goldblatt
2017-12-14 12:44 ` Edward Napierala
2017-12-14 13:15 ` Michal Hocko
2017-12-14 14:54 ` Edward Napierala
2017-12-19 12:40 ` David Laight
2017-12-19 12:46 ` Michal Hocko
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