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SubjectRe: [RFC PATCH] sysfs: bin_attr permission checking
* 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


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