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

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Message-Id: <20110326000459.388812559@clark.kroah.org>
Date:	Fri, 25 Mar 2011 17:04:06 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	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: [34/35] fs: call security_d_instantiate in d_obtain_alias V2

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.

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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