Re: [RFC PATCH] sysfs: bin_attr permission checking

[Chris Wright <chrisw@sous-sol.org>]

* Greg KH (greg@kroah.com) wrote:
> On Wed, May 12, 2010 at 11:47:13AM -0700, Chris Wright wrote:
> > The PCI config space bin_attr read handler has a hardcoded CAP_SYS_ADMIN
> > check to verify privileges before allowing a user to read device
> > dependent config space.  This is meant to protect from an unprivileged
> > user potentially locking up the box.
> > 
> > When assigning a PCI device directly to a guest with libvirt and KVM, the 
> > sysfs config space file is chown'd to the user that the KVM guest will
> > run as.  The guest needs to have full access to the device's config
> > space since it's responsible for driving the device.  However, despite
> > being the owner of the sysfs file, the CAP_SYS_ADMIN check will not
> > allow read access beyond the config header.
> > 
> > This patch adds a new bin_attr->read_file() callback which adds a struct
> > file to the normal argument list.  This allows an implementation such as
> > PCI config space bin_attr read_file handler to check both inode
> > permission as well as privileges to determine whether to allow
> > privileged actions through the handler.
> 
> Ick, this is all because we like showing different information if the
> user is "privileged or not" :(

yup

> Turns out, that this probably isn't the best user api to implement,
> remind me never to do that again...

Yeah, it's challenging to deal with.  Alternative here is a new config
sysfs entry that doesn't have this 'feature'.  (I looked into trying to
allow manageing the internal capable() check externally, not so pretty).

> > This is just RFC, although I've tested that it does allow the chown +
> > read to work as expected.  Any other ideas of how to handle this are
> > welcome.
> 
> Can we just pass in the 'file' for all users of the bin files instead of
> the dentry? 

The dentry doesn't currently go beyond sysfs/bin.c.  So, yes, I pushed
'file' through to last level in bin.c before ->read(), and can certinaly
just push through to ->read() as well.

>  You can always get the dentry from the file (as your patch
> showes), and there isn't that many users of this interface.  I'd really
> rather not have two different types of callbacks here.

Absolutely, this is just RFC (i.e. quicker to compile and test).  What
about write()?

thanks,
-chris