nfsd: fix oops on access from high-numbered ports

This bug was always here, but before my commit 6fa02839bf9412e18e77
("recheck for secure ports in fh_verify"), it could only be triggered by
failure of a kmalloc(). After that commit it could be triggered by a
client making a request from a non-reserved port for access to an export
marked "secure". (Exports are "secure" by default.)

The result is a struct svc_export with a reference count one too low,
resulting in likely oopses next time the export is accessed.

The reference counting here is not straightforward; a later patch will
clean up fh_verify().

Thanks to Lukas Hejtmanek for the bug report and followup.

Signed-off-by: J. Bruce Fields <[e-mail]>
Cc: Lukas Hejtmanek <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/nfsd/nfsfh.c

···
 		fhp->fh_dentry = dentry;
 		fhp->fh_export = exp;
 		nfsd_nr_verified++;
+		cache_get(&exp->h);
 	} else {
 		/*
 		 * just rechecking permissions
···
 		dprintk("nfsd: fh_verify - just checking\n");
 		dentry = fhp->fh_dentry;
 		exp = fhp->fh_export;
+		cache_get(&exp->h);
 		/*
 		 * Set user creds for this exportpoint; necessary even
 		 * in the "just checking" case because this may be a
···
 		if (error)
 			goto out;
 	}
-	cache_get(&exp->h);
-
 
 	error = nfsd_mode_check(rqstp, dentry->d_inode->i_mode, type);
 	if (error)