linux-kernel - Re: [PATCH v2] smackfs: restrict bytes count in smackfs write functions

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Date:   Thu, 28 Jan 2021 19:27:21 +0600
From:   Sabyrzhan Tasbolatov <snovitoll@...il.com>
To:     penguin-kernel@...ove.sakura.ne.jp
Cc:     andreyknvl@...gle.com, casey@...aufler-ca.com, jmorris@...ei.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, mhocko@...e.com,
        serge@...lyn.com, snovitoll@...il.com,
        syzbot+a71a442385a0b2815497@...kaller.appspotmail.com
Subject: Re: [PATCH v2] smackfs: restrict bytes count in smackfs write functions

> >  	/*
> > +	 * No partial write.
> >  	 * Enough data must be present.
> >  	 */
> >  	if (*ppos != 0)
> >  		return -EINVAL;
> > +	if (count == 0 || count > PAGE_SIZE)
> > +		return -EINVAL;
> >  
> >  	data = memdup_user_nul(buf, count);
> >  	if (IS_ERR(data))
> > 
> 
> Doesn't this change break legitimate requests like
> 
>   char buffer[20000];
> 
>   memset(buffer, ' ', sizeof(buffer));
>   memcpy(buffer + sizeof(buffer) - 10, "foo", 3);
>   write(fd, buffer, sizeof(buffer));
> 
> ?

It does, in this case. Then I need to patch another version with
whitespace stripping before, after label. I just followed the same thing
that I see in security/selinux/selinuxfs.c sel_write_enforce() etc.

It has the same memdup_user_nul() and count >= PAGE_SIZE check prior to that.