linux-kernel - Re: [34/35] fs: call security_d_instantiate in d_obtain

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Date:	Fri, 25 Mar 2011 17:24:56 -0700
From:	Casey Schaufler <casey@...aufler-ca.com>
To:	Greg KH <gregkh@...e.de>
CC:	linux-kernel@...r.kernel.org, stable@...nel.org,
	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Josef Bacik <josef@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Chuck Ebbert <cebbert@...hat.com>
Subject: Re: [34/35] fs: call security_d_instantiate in d_obtain_alias V2

On 3/25/2011 5:04 PM, Greg KH wrote:
> 2.6.33-longterm review patch.  If anyone has any objections, please let us know.
>
> ------------------
>
> From: Josef Bacik <josef@...hat.com>
>
> commit 24ff6663ccfdaf088dfa7acae489cb11ed4f43c4 upstream.
>
> 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.

Not to be a bother, but did you try this with Smack as well as SELinux?
Smack should be fine with the change, but if you're not going to try
Smack I need to know.

> Signed-off-by: Josef Bacik <josef@...hat.com>
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> Cc: Chuck Ebbert <cebbert@...hat.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
>
> ---
>  fs/dcache.c |    3 +++
>  1 file changed, 3 insertions(+)
>
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1176,9 +1176,12 @@ struct dentry *d_obtain_alias(struct ino
>  	spin_unlock(&tmp->d_lock);
>  
>  	spin_unlock(&dcache_lock);
> +	security_d_instantiate(tmp, inode);
>  	return tmp;
>  
>   out_iput:
> +	if (res && !IS_ERR(res))
> +		security_d_instantiate(res, inode);
>  	iput(inode);
>  	return res;
>  }
>
>
> --
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