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Date:   Mon, 8 Jun 2020 09:49:13 -0700
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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)
From:   sdf@google.com
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Vegard Nossum <vegard.nossum@oracle.com>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>, bpf <bpf@vger.kernel.org>,
        Andrey Ignatov <rdna@fb.com>
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On 06/08, Alexei Starovoitov wrote:
> On Mon, Jun 8, 2020 at 6:05 AM Christoph Hellwig <hch@lst.de> wrote:
> >
> > 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.

> adding Stanislav. I think he's looking into this already.
Yeah, I'm looking at it from the get/setsockopt point of view.
I'm currently trying to bypass allocating a buffer if it's greater
than PAGE_SIZE.
I suppose for sysctls we should try to do something similar?