Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 13 Nov 2018 17:55:58 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Ben Woodard <woodard@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] exec: increase BINPRM_BUF_SIZE to 256
Message-ID: <20181113165557.GG30990@redhat.com>
References: <20181112160931.GA28463@redhat.com>
 <20181112160956.GA28472@redhat.com>
 <20181112155248.4dde2613979f4c176565629e@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20181112155248.4dde2613979f4c176565629e@linux-foundation.org>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 11/12, Andrew Morton wrote:

> On Mon, 12 Nov 2018 17:09:56 +0100 Oleg Nesterov <oleg@redhat.com> wrote:
>
> >  /* sizeof(linux_binprm->buf) */
> > -#define BINPRM_BUF_SIZE 128
> > +#define BINPRM_BUF_SIZE 256
> >
> >  #endif /* _UAPI_LINUX_BINFMTS_H */
>
> It does seem a rather silly restriction, and it's tempting to suggest
> reworking the code so that linux_binprm.buf is dynamically sized to
> accommodate even ludicrously large strings.

I actually tried to do this ;)

Of course this is possible, but we need some limits anyway, we need to
read the file until we find '\n' or '\0' in kmalloc/vmalloc'ed buffer,
then split and copy the strings to bprm->vma in reverse order.

So I decided to make the trivial change for now and (hopefully) forget
about this problem.

> However it would be basically cost-free to increase
> BINPRM_BUF_SIZE up to the point where sizeof(struct linux_binprm) ==
> PAGE_SIZE?

I don't think we should take sizeof(struct linux_binprm) into account, the
new members can come at any time and we can never decrease BINPRM_BUF_SIZE.

Oleg.