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Date: Wed, 21 Feb 2018 22:39:38 +0300
From: Alexey Dobriyan <adobriyan@gmail.com>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/5] proc: make /proc/*/cmdline go through LSM
Message-ID: <20180221193938.GA28799@avx2>
References: <20180221192339.GA28548@avx2>
 <CAHp75VdaxkMn2UxURSB4fWep=iv4CRAkmEtdKBp1HKyxtW2tTg@mail.gmail.com>
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On Wed, Feb 21, 2018 at 09:28:41PM +0200, Andy Shevchenko wrote:
> On Wed, Feb 21, 2018 at 9:23 PM, Alexey Dobriyan <adobriyan@gmail.com> wrote:
> > /proc/*/cmdline is not different from /proc/*/environ as it accesses
> > target task's memory (and can access the very same region of memory)
> > but it doesn't go through ptrace_may_access() and thus doesn't go through LSM.
> 
> > +static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
> > +{
> > +       struct mm_struct *mm = proc_mem_open(inode, mode);
> > +
> > +       if (IS_ERR(mm))
> > +               return PTR_ERR(mm);
> 
> So, is it possible to have it NULL?..

I haven't looked closely, but if kernel thread is accesses then yes.

Regardless, patch only moves function so that code compiles, untangling
this little mess is separate adventure.

> > +static int mem_release(struct inode *inode, struct file *file)
> > +{
> > +       struct mm_struct *mm = file->private_data;
> 
> > +       if (mm)
> 
> ...or I don't get this check.
> 
> > +               mmdrop(mm);
> > +       return 0;
> > +}

It should trigger if kernel thread is accessed.