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

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Message-ID: <CAG48ez36xJ9UA8gWef3+1rHQwob5nb8WP3RqnbT8GEOV9Z38jA@mail.gmail.com>
Date:   Mon, 27 May 2019 15:38:26 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>,
        Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] binfmt_flat: make load_flat_shared_library() work

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.