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

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Message-Id: <20190525144304.e2b9475a18a1f78a964c5640@linux-foundation.org>
Date:   Sat, 25 May 2019 14:43:04 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, 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 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...