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

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Message-ID: <20101117191817.GA26575@fieldses.org>
Date:	Wed, 17 Nov 2010 14:18:17 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Josef Bacik <josef@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, eparis@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: call security_d_instantiate in d_obtain_alias

On Wed, Nov 17, 2010 at 12:51:03PM -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.  With this patch I'm no longer seeing these errant
> -EACCESS return values.  Thanks,

Possibly dumb question: Is there still a small race here?  Is it
possible for another nfsd thread to find the new alias on the list while
this thread is still:

> 
> Signed-off-by: Josef Bacik <josef@...hat.com>
> ---
>  fs/dcache.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 23702a9..890a59e 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1201,6 +1201,7 @@ struct dentry *d_obtain_alias(struct inode *inode)
>  	spin_unlock(&tmp->d_lock);
>  
>  	spin_unlock(&dcache_lock);

... right here, so that that other nfsd thread still ends up trying to
do something with a dentry that hasn't had security_d_instantiate called
on it yet?

> +	security_d_instantiate(tmp, inode);
>  	return tmp;
>  
>   out_iput:
> -- 

Or does something else prevent that?

--b.
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