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Date:   Mon, 8 Jun 2020 15:05:03 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Vegard Nossum <vegard.nossum@oracle.com>
Cc:     Christoph Hellwig <hch@lst.de>, Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        linux-kernel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
        bpf@vger.kernel.org, Andrey Ignatov <rdna@fb.com>
Subject: Re: WARNING: CPU: 1 PID: 52 at mm/page_alloc.c:4826
 __alloc_pages_nodemask (Re: [PATCH 5/5] sysctl: pass kernel
 pointers to ->proc_handler)
Message-ID: <20200608130503.GA22898@lst.de>
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On Mon, Jun 08, 2020 at 09:45:49AM +0200, Vegard Nossum wrote:
> Just a test case.
>
> Allowing the kernel to allocate an unbounded amount of memory on behalf
> of userspace is an easy DOS.
>
> All the length checks were already in there, e.g.
>
>  static int cmm_timeout_handler(struct ctl_table *ctl, int write,
>                               void __user *buffer, size_t *lenp, loff_t 
> *ppos)
>  {
>         char buf[64], *p;
> [...]
>                 len = min(*lenp, sizeof(buf));
>                 if (copy_from_user(buf, buffer, len))
>                         return -EFAULT;

Doesn't help if we don't know the exact limit yet.  But we can put
some arbitrary but reasonable limit like KMALLOC_MAX_SIZE on the
sysctls and see if this sticks.