Re: [PATCH] binfmt_flat: make load_flat_shared_library() work

From: Arnd Bergmann
Date: Wed May 29 2019 - 08:09:06 EST

On Tue, May 28, 2019 at 12:56 PM Greg Ungerer <gerg@xxxxxxxxxxxxxx> wrote:
> On 27/5/19 11:38 pm, Jann Horn wrote:
> > On Sat, May 25, 2019 at 11:43 PM Andrew Morton
> > <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >> On Fri, 24 May 2019 22:18:17 +0200 Jann Horn <jannh@xxxxxxxxxx> wrote:
> >>> load_flat_shared_library() is broken: It only calls load_flat_file() if
> >>> prepare_binprm() returns zero, but prepare_binprm() returns the number of
> >>> bytes read - so this only happens if the file is empty.
> >>
> >> ouch.
> >>
> >>> Instead, call into load_flat_file() if the number of bytes read is
> >>> non-negative. (Even if the number of bytes is zero - in that case,
> >>> load_flat_file() will see nullbytes and return a nice -ENOEXEC.)
> >>>
> >>> In addition, remove the code related to bprm creds and stop using
> >>> prepare_binprm() - this code is loading a library, not a main executable,
> >>> and it only actually uses the members "buf", "file" and "filename" of the
> >>> linux_binprm struct. Instead, call kernel_read() directly.
> >>>
> >>> Cc: stable@xxxxxxxxxxxxxxx
> >>> Fixes: 287980e49ffc ("remove lots of IS_ERR_VALUE abuses")
> >>> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> >>> ---
> >>> I only found the bug by looking at the code, I have not verified its
> >>> existence at runtime.
> >>> Also, this patch is compile-tested only.
> >>> It would be nice if someone who works with nommu Linux could have a
> >>> look at this patch.
> >>
> >> 287980e49ffc was three years ago! Has it really been broken for all
> >> that time? If so, it seems a good source of freed disk space...
> >
> > Maybe... but I didn't want to rip it out without having one of the
> > maintainers confirm that this really isn't likely to be used anymore.
> I have not used shared libraries on m68k non-mmu setups for
> a very long time. At least 10 years I would think.

I think Emcraft have a significant customer base running ARM NOMMU
Linux, I wonder whether they would have run into this (adding
Sergei to Cc).
My suspicion is that they use only binfmt-elf-fdpic, not binfmt-flat.

The only architectures I see that enable binfmt-flat are sh, xtensa
and h8300, but only arch/sh uses CONFIG_BINFMT_SHARED_FLAT
for a few machine specific configurations, and I'm in turn fairly sure
those machines have not run a recent kernel in many years.

The one SH nommu platform likely to have users is j2, and that is
probably always used with musl-libc with elf-fdpic (given that
Rich Felker maintains both the kernel port and the library).