Re: [RFC] revoke(2) and generic handling of things like remove_proc_entry()

[Hannes Frederic Sowa]

On Fri, Apr 05, 2013 at 05:29:32AM +0100, Al Viro wrote:
> 4) nasty semantics issue - mmap() vs. revoke (of any sort, including
> remove_proc_entry(), etc.).  Suppose a revokable file had been mmapped;
> now it's going away.  What should we do to its VMAs?  Right now sysfs
> and procfs get away with that, but only because there's only one thing
> that has ->mmap() there - /proc/bus/pci and sysfs equivalents.  I've
> no idea how does pci_mmap_page_range() interact with PCI hotplug (and
> I'm not at all sure that whatever it does isn't racy wrt device removal),
> but I suspect that it strongly depends on lack of ->fault() for those
> VMAs, which makes killing all PTEs pointing to pages in question enough.
> How generic do we want to make it?  Anybody wanting to add more files
> that could be mmapped in procfs/sysfs/debugfs deserves to be hurt, but
> if we start playing with revoke(2), restriction might become inconvenient.
> I'm not sure what kind of behaviour do we want there - *BSD at least
> used to have revoke(2) only for character devices that had no mmap()...

I am seeing possible problems in software implementing their own memory
management ontop SIGSEGV e.g. java. I hope they sanely distinguish
between heap mappings and file mmaps.

FreeBSD allowes tearing down a mmap on MAC security relabel. Two possible
actions are available: SIGSEGV generation by tearing down the mapping
forcefully or enable some kind of copy-on-write semantics on revoke:

http://svnweb.freebsd.org/base/head/sys/security/mac/mac_process.c?revision=248084&view=markup

I like to see something like revoke being worked on, thanks!

Greetings,

  Hannes

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