linux-kernel - Re: [PATCH 1/5] fs: Verify access of user towards block device file when mounting

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Date:	Thu, 1 Oct 2015 07:55:08 -0500
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
	Neil Brown <neilb@...e.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Brian Norris <computersforpeace@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andy Lutomirski <luto@...capital.net>,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov,
	linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
	linux-bcache@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: [PATCH 1/5] fs: Verify access of user towards block device file
 when mounting

On Wed, Sep 30, 2015 at 07:42:15PM -0400, Mike Snitzer wrote:
> On Wed, Sep 30 2015 at  4:15pm -0400,
> Seth Forshee <seth.forshee@...onical.com> wrote:
> 
> > When mounting a filesystem on a block device there is currently
> > no verification that the user has appropriate access to the
> > device file passed to mount. This has not been an issue so far
> > since the user in question has always been root, but this must
> > be changed before allowing unprivileged users to mount in user
> > namespaces.
> > 
> > To fix this, add an argument to lookup_bdev() to specify the
> > required permissions. If the mask of permissions is zero, or
> > if the user has CAP_SYS_ADMIN, the permission check is skipped,
> > otherwise the lookup fails if the user does not have the
> > specified access rights for the inode at the supplied path.
> > 
> > Callers associated with mounting are updated to pass permission
> > masks to lookup_bdev() so that these mounts will fail for an
> > unprivileged user who lacks permissions for the block device
> > inode. All other callers pass 0 to maintain their current
> > behaviors.
> > 
> > Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
> > ---
> >  drivers/md/bcache/super.c |  2 +-
> >  drivers/md/dm-table.c     |  2 +-
> >  drivers/mtd/mtdsuper.c    |  6 +++++-
> >  fs/block_dev.c            | 18 +++++++++++++++---
> >  fs/quota/quota.c          |  2 +-
> >  include/linux/fs.h        |  2 +-
> >  6 files changed, 24 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> > index e76ed003769e..35bb3ea4cbe2 100644
> > --- a/drivers/md/dm-table.c
> > +++ b/drivers/md/dm-table.c
> > @@ -380,7 +380,7 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> >  	BUG_ON(!t);
> >  
> >  	/* convert the path to a device */
> > -	bdev = lookup_bdev(path);
> > +	bdev = lookup_bdev(path, 0);
> >  	if (IS_ERR(bdev)) {
> >  		dev = name_to_dev_t(path);
> >  		if (!dev)
> 
> Given dm_get_device() is passed @mode why not have it do something like
> you did in blkdev_get_by_path()? e.g.:

I only dealt with code related to mounting in this patch since that's
what I'm working on. I have it on my TODO list to consider converting
other callers of lookup_bdev. But if you're sure doing so makes sense
for dm_get_device and that it won't cause regressions then I could add a
patch for it.

Thanks,
Seth

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