linux-kernel - Re: [PATCH 2/2] exec: increase BINPRM_BUF

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Date:   Tue, 13 Nov 2018 17:55:58 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Ben Woodard <woodard@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] exec: increase BINPRM_BUF_SIZE to 256

On 11/12, Andrew Morton wrote:

> On Mon, 12 Nov 2018 17:09:56 +0100 Oleg Nesterov <oleg@...hat.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.