linux-kernel - Re: [PATCH] fs: call security_d_instantiate in d_obtain

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Message-ID: <20101121154402.GA20863@dhcp231-156.rdu.redhat.com>
Date:	Sun, 21 Nov 2010 10:44:04 -0500
From:	Josef Bacik <josef@...hat.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Josef Bacik <josef@...hat.com>, linux-fsdevel@...r.kernel.org,
	eparis@...hat.com, linux-kernel@...r.kernel.org, sds@...ho.nsa.gov,
	selinux@...ho.nsa.gov
Subject: Re: [PATCH] fs: call security_d_instantiate in d_obtain_alias V2

On Sat, Nov 20, 2010 at 09:59:45PM -0500, J. Bruce Fields wrote:
> On Fri, Nov 19, 2010 at 05:35:52PM -0500, J. Bruce Fields wrote:
> > On Thu, Nov 18, 2010 at 08:52:55PM -0500, Josef Bacik wrote:
> > > While trying to track down some NFS problems with BTRFS, I kept noticing I was
> > > getting -EACCESS for no apparent reason.  Eric Paris and printk() helped me
> > > figure out that it was SELinux that was giving me grief, with the following
> > > denial
> > > 
> > > type=AVC msg=audit(1290013638.413:95): avc:  denied  { 0x800000 } for  pid=1772
> > > comm="nfsd" name="" dev=sda1 ino=256 scontext=system_u:system_r:kernel_t:s0
> > > tcontext=system_u:object_r:unlabeled_t:s0 tclass=file
> > > 
> > > Turns out this is because in d_obtain_alias if we can't find an alias we create
> > > one and do all the normal instantiation stuff, but we don't do the
> > > security_d_instantiate.
> > > 
> > > Usually we are protected from getting a hashed dentry that hasn't yet run
> > > security_d_instantiate() by the parent's i_mutex, but obviously this isn't an
> > > option there, so in order to deal with the case that a second thread comes in
> > > and finds our new dentry before we get to run security_d_instantiate(), we go
> > > ahead and call it if we find a dentry already.  Eric assures me that this is ok
> > > as the code checks to see if the dentry has been initialized already so calling
> > > security_d_instantiate() against the same dentry multiple times is ok.  With
> > > this patch I'm no longer getting errant -EACCESS values.
> > 
> > Thanks, I can't see any reason that wouldn't work.
> 
> (FWIW, I also ran my usual nfs regression tests with this applied.  They
> don't exercise the problem you were seeing, but maybe it's at least some
> sort of sanity check.)
>

Oh I should have mentioned the testing I gave it.  With BTRFS I could regularly
reproduce this by doing the following

SERVER
mount /dev/sda1 /mnt/btrfs
btrfs subvol create /mnt/btrfs/foo
cp mutt.tar.gz /mnt/btrfs/foo

CLIENT
mount server:/mnt/btrfs /mnt/test
cd /mnt/test/foo
tar xzvf mutt.tar.gz
cd mutt
ls

SERVER
echo 3 > /proc/sys/vm/drop_caches

CLIENT
ls

and bam I'd get -EACCESS every time.  I did this a bunch of times to make sure
it didn't happen anymore plus variations of the above.  Thanks,

Josef 
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