Re: CVE-2014-8159 kernel: infiniband: uverbs: unprotected physical memory access

[Yann Droneaud]

Hi Haggai,

Le jeudi 02 avril 2015 Ã 18:18 +0300, Haggai Eran a Ãcrit :
> On 02/04/2015 16:30, Yann Droneaud wrote:
> > Hi,
> > 
> > Le jeudi 02 avril 2015 Ã 10:52 +0000, Shachar Raindel a Ãcrit :
> >>> -----Original Message-----
> >>> From: Yann Droneaud [mailto:ydroneaud@xxxxxxxxxx]
> >>> Sent: Thursday, April 02, 2015 1:05 PM
> >>> Le mercredi 18 mars 2015 Ã 17:39 +0000, Shachar Raindel a Ãcrit :
> > 
> >>>> +	/*
> >>>> +	 * If the combination of the addr and size requested for this
> >>> memory
> >>>> +	 * region causes an integer overflow, return error.
> >>>> +	 */
> >>>> +	if ((PAGE_ALIGN(addr + size) <= size) ||
> >>>> +	    (PAGE_ALIGN(addr + size) <= addr))
> >>>> +		return ERR_PTR(-EINVAL);
> >>>> +
> >>>
> >>> Can access_ok() be used here ?
> >>>
> >>>          if (!access_ok(writable ? VERIFY_WRITE : VERIFY_READ,
> >>>                         addr, size))
> >>>                   return ERR_PTR(-EINVAL);
> >>>
> >>
> >> No, this will break the current ODP semantics.
> >>
> >> ODP allows the user to register memory that is not accessible yet.
> >> This is a critical design feature, as it allows avoiding holding
> >> a registration cache. Adding this check will break the behavior,
> >> forcing memory to be all accessible when registering an ODP MR.
> >>
> > 
> > Where's the check for the range being in userspace memory space,
> > especially for the ODP case ?
> > 
> > For non ODP case (eg. plain old behavior), does get_user_pages()
> > ensure the requested pages fit in userspace region on all 
> > architectures ? I think so.
> 
> Yes, get_user_pages will return a smaller amount of pages than requested
> if it encounters an unmapped region (or a region without write
> permissions for write requests). If this happens, the loop in
> ib_umem_get calls get_user_pages again with the next set of pages, and
> this time if it the first page still cannot be mapped an error is returned.
> 
> > 
> > In ODP case, I'm not sure such check is ever done ?
> 
> In ODP, we also call get_user_pages, but only when a page fault occurs
> (see ib_umem_odp_map_dma_pages()). This allows the user to pre-register
> a memory region that contains unmapped virtual space, and then mmap
> different files into that area without needing to re-register.
> 

OK, thanks for the description.

> > (Aside, does it take special mesure to protect shared mapping from
> > being read and/or *written* ?)
> 
> I'm not sure I understand the question. Shared mappings that the process
> is allowed to read or write are also allowed for the HCA (specifically,
> to local and remote operations the same process performs using the HCA),
> provided the application has registered their virtual address space as a
> memory region.
> 

I was refering to description of get_user_pages():

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/mm/gup.c?id=v4.0-rc6#n765

 * @force:	whether to force access even when user mapping is currently
 *		protected (but never forces write access to shared mapping).

But since ib_umem_odp_map_dma_pages() use get_user_pages() with force
argument set to 0, it's OK.

Another related question: as the large memory range could be registered 
by user space with ibv_reg_mr(pd, base, size, IB_ACCESS_ON_DEMAND), 
what's prevent the kernel to map a file as the result of mmap(0, ...)
in this  region, making it available remotely through IBV_WR_RDMA_READ /
IBV_WR_RDMA_WRITE ?

Again, thanks for the information I was missing to understand how ODP is
checking the memory ranges.

Regards.

-- 
Yann Droneaud
OPTEYA


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