# Re: \[PATCH v2 0/7\] Remove in-tree usage of MAP\_DENYWRITE

- From: Christian König \<christian.koenig@amd.com\>
- Date: 2021-08-17 11:01:37 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2021/08/212563
- Thread: https://ratatoskr.run/linux-unionfs/2021/08/212501/t.md

```text
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
> behavior.
>
> 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
>     grasp.
> -- 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 [ref] to the series.

Thanks,
Christian.

>
> [1] https://lore.kernel.org/r/20210423131640.20080-1-david@redhat.com/
> [2] https://lore.kernel.org/r/YNHXzBgzRrZu1MrD@miu.piliscsaba.redhat.com/
> [3] https://lkml.kernel.org/r/20210812084348.6521-1-david@redhat.com
>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Thomas Gleixner <redacted>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Alexey Dobriyan <redacted>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Jiri Olsa <redacted>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Sergey Senozhatsky <redacted>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Cc: Kees Cook <redacted>
> Cc: "Eric W. Biederman" <redacted>
> Cc: Greg Ungerer <gerg@linux-m68k.org>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Vlastimil Babka <redacted>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: Chinwen Chang <redacted>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Cc: Huang Ying <redacted>
> Cc: Jann Horn <jannh@google.com>
> Cc: Feng Tang <redacted>
> Cc: Kevin Brodsky <redacted>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Shawn Anastasio <redacted>
> Cc: Steven Price <steven.price@arm.com>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Christian Brauner <redacted>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Gabriel Krisman Bertazi <redacted>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Shakeel Butt <redacted>
> Cc: Marco Elver <elver@google.com>
> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
> Cc: Nicolas Viennot <redacted>
> Cc: Thomas Cedeno <redacted>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Miklos Szeredi <miklos@szeredi.hu>
> Cc: Chengguang Xu <redacted>
> Cc: "Christian König" <ckoenig.leichtzumerken@gmail.com>
> Cc: Florian Weimer <redacted>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: David Laight <redacted>
> Cc: <redacted>
> Cc: <redacted>
> Cc: x86@kernel.org
> Cc: <redacted>
> Cc: <redacted>
>
> David Hildenbrand (7):
>    binfmt: don't use MAP_DENYWRITE when loading shared libraries via
>      uselib()
>    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

```