Re: kernel BUG at mm/gup.c:LINE!

From: Michal Hocko <mhocko@kernel.org>
To: Oscar Salvador <osalvador@techadventures.net>
Cc: Zi Yan <zi.yan@cs.rutgers.edu>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	syzbot <syzbot+5dcb560fe12aa5091c06@syzkaller.appspotmail.com>,
	akpm@linux-foundation.org, aneesh.kumar@linux.vnet.ibm.com,
	dan.j.williams@intel.com, kirill.shutemov@linux.intel.com,
	linux-mm@kvack.org, mst@redhat.com,
	syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk,
	ying.huang@intel.com
Subject: Re: kernel BUG at mm/gup.c:LINE!
Date: Thu, 5 Jul 2018 08:44:19 +0200	[thread overview]
Message-ID: <20180705064335.GA32658@dhcp22.suse.cz> (raw)
In-Reply-To: <20180704151529.GA23317@techadventures.net>

On Wed 04-07-18 17:15:29, Oscar Salvador wrote:
> > 
> > Not really. vm_brk_flags does call mm_populate for mlocked brk which is
> > the case for mlockall. I do not see any len sanitization in that path.
> > Well do_brk_flags does the roundup. I think we should simply remove the
> > bug on and round up there. mm_populate is an internal API and we should
> > trust our callers.
> > 
> > Anyway, the minimum fix seems to be the following (untested):
> > 
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 9859cd4e19b9..56ad19cf2aea 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -186,8 +186,8 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
> >  	return next;
> >  }
> >  
> > -static int do_brk(unsigned long addr, unsigned long len, struct list_head *uf);
> > -
> > +static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags,
> > +		struct list_head *uf);
> >  SYSCALL_DEFINE1(brk, unsigned long, brk)
> >  {
> >  	unsigned long retval;
> > @@ -245,7 +245,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
> >  		goto out;
> >  
> >  	/* Ok, looks good - let it rip. */
> > -	if (do_brk(oldbrk, newbrk-oldbrk, &uf) < 0)
> > +	if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
> >  		goto out;
> >  
> >  set_brk:
> > @@ -2939,12 +2939,6 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
> >  	pgoff_t pgoff = addr >> PAGE_SHIFT;
> >  	int error;
> >  
> > -	len = PAGE_ALIGN(request);
> > -	if (len < request)
> > -		return -ENOMEM;
> > -	if (!len)
> > -		return 0;
> > -
> >  	/* Until we need other flags, refuse anything except VM_EXEC. */
> >  	if ((flags & (~VM_EXEC)) != 0)
> >  		return -EINVAL;
> > @@ -3016,18 +3010,20 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
> >  	return 0;
> >  }
> >  
> > -static int do_brk(unsigned long addr, unsigned long len, struct list_head *uf)
> > -{
> > -	return do_brk_flags(addr, len, 0, uf);
> > -}
> > -
> > -int vm_brk_flags(unsigned long addr, unsigned long len, unsigned long flags)
> > +int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
> >  {
> >  	struct mm_struct *mm = current->mm;
> > +	unsigned long len;
> >  	int ret;
> >  	bool populate;
> >  	LIST_HEAD(uf);
> >  
> > +	len = PAGE_ALIGN(request);
> > +	if (len < request)
> > +		return -ENOMEM;
> > +	if (!len)
> > +		return 0;
> > +
> >  	if (down_write_killable(&mm->mmap_sem))
> >  		return -EINTR;
> 
> I gave this patch a try but the system doesn't boot.
> Unfortunately, I don't have the stacktrace on hand, but I'll get back to it tomorrow.

This is more than unexpected. The patch merely move the alignment check
up. I will try to investigate some more but I am off for next four days
and won't be online most of the time.

Btw. does the same happen if you keep do_brk helper and add the length
sanitization there as well?
-- 
Michal Hocko
SUSE Labs