linux-kernel - Re: Unmapping of UIO logical memory causing a trace

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 11 Aug 2015 14:45:45 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Ankit Jindal <ajindal@....com>
Cc:	linux-kernel@...r.kernel.org, "Hans J. Koch" <hjk@...sjkoch.de>,
	Pranavkumar Sawargaonkar <psawargaonkar@....com>,
	Jitendra Kanitkar <jkanitkar@....com>,
	Tushar Jagad <tjagad@....com>
Subject: Re: Unmapping of UIO logical memory causing a trace

On Tue, Aug 11, 2015 at 04:39:08PM +0530, Ankit Jindal wrote:
> Hi,
> 
> We have observed an issue where kmalloc of a small sized memory causes
> an occasional trace when unmapping the mmaped memory via UIO framework
> This trace is coming when kernel sees a negative value in
> page->_mapcount. Trace is pasted at the end of the mail.
> 
> After debugging this issue further, we realized following sequence
> occurs when kmalloc is used to allocate small memory using slub
> allocator:
> 1. Frozen bit (msb) of the page from which memory has been allocated
> is set (which is an union with _mapcount).
> 2. If there are free objects in the the same page then this frozen bit
> remains set even after kernel boots completely.
> 3. When user space calls unmap of this memory, vma_unmap_single()
> treats the _mapcount as a negative (as frozen bit is set), causing a
> trace.
> 
> We are not sure whether exposing kernel memory of size
> less than PAGE_SIZE via UIO is a valid use case ? In case this is an invalid
> use case then shouldn't the UIO framework restrict mapping of non
> PAGE_SIZE aligned memory and size not in order of PAGE_SIZE.

We've had a few discussions about this in the past, and one proposed
patch which had to be reverted because it broke some working systems, so
it's a messy thing.

What UIO driver are you using that causes this behavior?

thanks,

greg k-h
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/