Re: [PATCH 3/6] mremap: Don't allow MREMAP_DONTUNMAP on special_mappings and aio

From: Dmitry Safonov <dima@arista.com>
To: Brian Geffon <bgeffon@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Dmitry Safonov <0x7f454c46@gmail.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Hugh Dickins <hughd@google.com>, Ingo Molnar <mingo@redhat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Minchan Kim <minchan@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Vlastimil Babka <vbabka@suse.cz>, Will Deacon <will@kernel.org>,
	linux-aio@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH 3/6] mremap: Don't allow MREMAP_DONTUNMAP on special_mappings and aio
Date: Mon, 28 Dec 2020 19:43:16 +0000	[thread overview]
Message-ID: <85e469ee-9986-30cf-09d2-832eb8a61291@arista.com> (raw)
In-Reply-To: <d5d608ba-a25c-d3a2-b0f4-c97437a6dab1@arista.com>

On 12/28/20 7:33 PM, Dmitry Safonov wrote:
> [I moved your reply to avoid top-posting]
> 
> On 12/28/20 6:03 PM, Brian Geffon wrote:
>> On Mon, Oct 12, 2020 at 6:34 PM Dmitry Safonov <dima@arista.com> wrote:
>>>
>>> As kernel expect to see only one of such mappings, any further
>>> operations on the VMA-copy may be unexpected by the kernel.
>>> Maybe it's being on the safe side, but there doesn't seem to be any
>>> expected use-case for this, so restrict it now.
>>>
>>> Fixes: commit e346b3813067 ("mm/mremap: add MREMAP_DONTUNMAP to mremap()")
>>> Signed-off-by: Dmitry Safonov <dima@arista.com>
>>
>> I don't think this situation can ever happen MREMAP_DONTUNMAP is
>> already restricted to anonymous mappings (defined as not having
>> vm_ops) and vma_to_resize checks that the mapping is anonymous before
>> move_vma is called.
> 
> I've looked again now, I think it is possible. One can call
> MREMAP_DONTUNMAP without MREMAP_FIXED and without resizing. So that the
> old VMA is copied at some free address.
> 
> The calltrace would be: mremap()=>move_vma()
> [under if (flags & MREMAP_MAYMOVE)].
> 
> On the other side I agree with you that the fix could have been better
> if I realized the semantics that MREMAP_DONTUNMAP should only work with
> anonymous mappings.
> 
> Probably, a better fix would be to move
> :       if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) ||
> :                       vma->vm_flags & VM_SHARED))
> :               return ERR_PTR(-EINVAL);
> 
> from vma_to_resize() into the mremap() syscall directly.
> What do you think?

Ok, I've misread the code now, it checks vma_to_resize() before.
I'll send a revert to this one.

Thanks for noticing,
          Dima