linux-kernel - Re: [PATCH] binfmt_flat: make load_flat_shared

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Date:   Wed, 29 May 2019 14:05:21 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Greg Ungerer <gerg@...ux-m68k.org>
Cc:     Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Linux/m68k" <linux-m68k@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Sergei Poselenov <sposelenov@...raft.com>
Subject: Re: [PATCH] binfmt_flat: make load_flat_shared_library() work

On Tue, May 28, 2019 at 12:56 PM Greg Ungerer <gerg@...ux-m68k.org> wrote:
> On 27/5/19 11:38 pm, Jann Horn wrote:
> > On Sat, May 25, 2019 at 11:43 PM Andrew Morton
> > <akpm@...ux-foundation.org> wrote:
> >> On Fri, 24 May 2019 22:18:17 +0200 Jann Horn <jannh@...gle.com> 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@...r.kernel.org
> >>> Fixes: 287980e49ffc ("remove lots of IS_ERR_VALUE abuses")
> >>> Signed-off-by: Jann Horn <jannh@...gle.com>
> >>> ---
> >>> 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).

      Arnd