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

From: Arnd Bergmann <arnd@arndb.de>
To: Greg Ungerer <gerg@linux-m68k.org>
Cc: Jann Horn <jannh@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Nicolas Pitre <nicolas.pitre@linaro.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Linux/m68k <linux-m68k@vger.kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Sergei Poselenov <sposelenov@emcraft.com>
Subject: Re: [PATCH] binfmt_flat: make load_flat_shared_library() work
Date: Wed, 29 May 2019 14:05:21 +0200	[thread overview]
Message-ID: <CAK8P3a0b7MBn+84jh0Y2zhFLLAqZ2tMvFDFF9Kw=breRLH4Utg@mail.gmail.com> (raw)
In-Reply-To: <6956cfe5-90d4-aad4-48e3-66b0ece91fed@linux-m68k.org>

On Tue, May 28, 2019 at 12:56 PM Greg Ungerer <gerg@linux-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@linux-foundation.org> wrote:
> >> On Fri, 24 May 2019 22:18:17 +0200 Jann Horn <jannh@google.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@vger.kernel.org
> >>> Fixes: 287980e49ffc ("remove lots of IS_ERR_VALUE abuses")
> >>> Signed-off-by: Jann Horn <jannh@google.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