Re: [RFC PATCH v1 0/1] binfmt_elf: seal address zero

[Jeff Xu]

On Mon, Aug 5, 2024 at 2:01 PM Kees Cook <kees@xxxxxxxxxx> wrote:
>
> On Thu, Aug 01, 2024 at 05:08:32PM +0000, jeffxu@xxxxxxxxxxxx wrote:
> > From: Jeff Xu <jeffxu@xxxxxxxxxxxx>
> >
> > 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].
>
> For users, I didn't find much in a Debian Code Search:
> https://codesearch.debian.net/search?q=MMAP_PAGE_ZERO&literal=1&perpkg=1
>
> I see rr uses it in testing, and some utils have it as an option, so I
> think maybe just leave it supported.
>
Thanks for checking, I will add those in the V2 cover letter.

> >
> > 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.
>
> Yeah, this seems fine to me.
>
> >
> > [1] https://lore.kernel.org/lkml/CAHk-=whVa=nm_GW=NVfPHqcxDbWt4JjjK1YWb0cLjO4ZSGyiDA@xxxxxxxxxxxxxx/
> >
> > Jeff Xu (1):
> >   binfmt_elf: mseal address zero
> >
> >  fs/binfmt_elf.c    | 4 ++++
> >  include/linux/mm.h | 4 ++++
> >  mm/mseal.c         | 2 +-
> >  3 files changed, 9 insertions(+), 1 deletion(-)
> >
> > --
> > 2.46.0.rc1.232.g9752f9e123-goog
> >
>
> --
> Kees Cook