[v2] binfmt_elf: seal address zero

[PATCH v2 0/1] binfmt_elf: seal address zero

Posted by jeffxu@chromium.org 2 months, 3 weeks ago

From: Jeff Xu <jeffxu@chromium.org>

In load_elf_binary as part of the execve(),  when the current
task’s personality has MMAP_PAGE_ZERO set, the kernel allocates
one page at address 0. According to the comment:

/* Why this, you ask???  Well SVr4 maps page 0 as read-only,
    and some applications "depend" upon this behavior.
    Since we do not have the power to recompile these, we
     emulate the SVr4 behavior. Sigh. */

At one point, Linus suggested removing this [1].

Code search in debian didn't see much use of MMAP_PAGE_ZERO [2],
it exists in util and test (rr).

Sealing this is probably safe, the comment doesn’t say 
the app ever wanting to change the mapping to rwx. Sealing
also ensures that never happens.

[1] https://lore.kernel.org/lkml/CAHk-=whVa=nm_GW=NVfPHqcxDbWt4JjjK1YWb0cLjO4ZSGyiDA@mail.gmail.com/
[2] https://codesearch.debian.net/search?q=MMAP_PAGE_ZERO&literal=1&perpkg=1&page=1

Jeff Xu (1):
  binfmt_elf: mseal address zero

 fs/binfmt_elf.c    |  5 +++++
 include/linux/mm.h | 10 ++++++++++
 mm/mseal.c         |  2 +-
 3 files changed, 16 insertions(+), 1 deletion(-)

-- 
2.46.0.rc2.264.g509ed76dc8-goog

Re: [PATCH v2 0/1] binfmt_elf: seal address zero

Posted by Kees Cook 2 months, 2 weeks ago

On Tue, 06 Aug 2024 21:49:26 +0000, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@chromium.org>
> 
> In load_elf_binary as part of the execve(),  when the current
> task’s personality has MMAP_PAGE_ZERO set, the kernel allocates
> one page at address 0. According to the comment:
> 
> /* Why this, you ask???  Well SVr4 maps page 0 as read-only,
>     and some applications "depend" upon this behavior.
>     Since we do not have the power to recompile these, we
>      emulate the SVr4 behavior. Sigh. */
> 
> [...]

I added the cover letter details to the commit log and changed pr_warn()
to pr_warn_ratelimited(), but otherwise, looked good.

Applied to for-next/execve, thanks!

[1/1] binfmt_elf: mseal address zero
      https://git.kernel.org/kees/c/44f65d900698

Take care,

-- 
Kees Cook