Re: [PATCH v2 0/7] Remove in-tree usage of MAP_DENYWRITE

From: Christian König
Date: Tue Aug 17 2021 - 07:01:38 EST

Am 16.08.21 um 21:48 schrieb David Hildenbrand:
This series removes all in-tree usage of MAP_DENYWRITE from the kernel
and removes VM_DENYWRITE. We stopped supporting MAP_DENYWRITE for
user space applications a while ago because of the chance for DoS.
The last renaming user is binfmt binary loading during exec and
legacy library loading via uselib().

With this change, MAP_DENYWRITE is effectively ignored throughout the
kernel. Although the net change is small (well, we actually add code and
comments), I think the cleanup in mmap() is quite nice.

There are some (minor) user-visible changes with this series:
1. We no longer deny write access to shared libaries loaded via legacy
uselib(); this behavior matches modern user space e.g., via dlopen().
2. We no longer deny write access to the elf interpreter after exec
completed, treating it just like shared libraries (which it often is).
3. We always deny write access to the file linked via /proc/pid/exe:
sys_prctl(PR_SET_MM_MAP/EXE_FILE) will fail if write access to the file
cannot be denied, and write access to the file will remain denied
until the link is effectivel gone (exec, termination,
sys_prctl(PR_SET_MM_MAP/EXE_FILE)) -- just as if exec'ing the file.

There is a related problem [2] with overlayfs, that should at least partly
be tackled by this series. I don't quite understand the interaction of
overlayfs and deny_write_access()/allow_write_access() at exec time:

If we end up denying write access to the wrong file and not to the
realfile, that would be fundamentally broken. We would have to reroute
our deny_write_access()/ allow_write_access() calls for the exec file to
the realfile -- but I leave figuring out the details to overlayfs guys, as
that would be a related but different issue.

There was a lengthy discussion in [3] whether to remove deny_write_access()
completely; however, if we decide to go that way, it would ideally be done
on top, because it could be that some applications even rely on the current

v1 -> v2:
- "kernel/fork: factor out replacing the current MM exe_file"
-- Call the function "replace_mm_exe_file()" instead
-- Add some doc, similar to set_mm_exe_file()
-- Update patch subject/description
- "kernel/fork: always deny write access to current MM exe_file"
-- Introduce dup_mm_exe_file()
-- Make set_mm_exe_file() return an error to make the code easier to
-- Improve comments
- Added ACKs
- Mention "sys_prctl(PR_SET_MM_MAP/EXE_FILE)" everywhere instead of
only "sys_prctl(PR_SET_MM_EXE_FILE)".

RFC -> v1:
- "binfmt: remove in-tree usage of MAP_DENYWRITE"
-- Add a note that this should fix part of a problem with overlayfs

This is unfortunately way beyond my understanding of the fs layer to actually review this.

But from the ten mile high view it looks like a really nice cleanup to me and makes my live in the DRM subsystem much easier.

Feel free to add a Acked-by: Christian König <christian.koenig@xxxxxxx> to the series.



Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
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Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Cc: Chinwen Chang <chinwen.chang@xxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
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Cc: Kevin Brodsky <Kevin.Brodsky@xxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
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Cc: "Christian König" <ckoenig.leichtzumerken@xxxxxxxxx>
Cc: Florian Weimer <fweimer@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: David Laight <David.Laight@xxxxxxxxxx>
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Cc: linux-mm@xxxxxxxxx

David Hildenbrand (7):
binfmt: don't use MAP_DENYWRITE when loading shared libraries via
kernel/fork: factor out replacing the current MM exe_file
kernel/fork: always deny write access to current MM exe_file
binfmt: remove in-tree usage of MAP_DENYWRITE
mm: remove VM_DENYWRITE
mm: ignore MAP_DENYWRITE in ksys_mmap_pgoff()
fs: update documentation of get_write_access() and friends

arch/x86/ia32/ia32_aout.c | 8 ++-
fs/binfmt_aout.c | 7 ++-
fs/binfmt_elf.c | 6 +--
fs/binfmt_elf_fdpic.c | 2 +-
fs/exec.c | 4 +-
fs/proc/task_mmu.c | 1 -
include/linux/fs.h | 19 ++++---
include/linux/mm.h | 4 +-
include/linux/mman.h | 4 +-
include/trace/events/mmflags.h | 1 -
kernel/events/core.c | 2 -
kernel/fork.c | 95 ++++++++++++++++++++++++++++++----
kernel/sys.c | 33 +-----------
lib/test_printf.c | 5 +-
mm/mmap.c | 29 ++---------
mm/nommu.c | 2 -
16 files changed, 119 insertions(+), 103 deletions(-)

base-commit: 7c60610d476766e128cc4284bb6349732cbd6606