nfsd: simplify fh_verify access checks

All nfsd security depends on the security checks in fh_verify, and
especially on nfsd_setuser().

It therefore bothers me that the nfsd_setuser call may be made from
three different places, depending on whether the filehandle has already
been mapped to a dentry, and on whether subtreechecking is in force.

Instead, make an unconditional call in fh_verify(), so it's trivial to
verify that the call always occurs.

That leaves us with a redundant nfsd_setuser() call in the subtreecheck
case--it needs the correct user set earlier in order to check execute
permissions on the path to this filehandle--but I'm willing to accept
that minor inefficiency in the subtreecheck case in return for more
straightforward permission checking.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfsfh.c

@@ -233,14 +233,6 @@ static __be32 nfsd_set_fh_dentry(struct svc_rqst *rqstp, struct svc_fh *fhp)
 		goto out;
 	}
 
-	if (exp->ex_flags & NFSEXP_NOSUBTREECHECK) {
-		error = nfsd_setuser_and_check_port(rqstp, exp);
-		if (error) {
-			dput(dentry);
-			goto out;
-		}
-	}
-
 	if (S_ISDIR(dentry->d_inode->i_mode) &&
 			(dentry->d_flags & DCACHE_DISCONNECTED)) {
 		printk("nfsd: find_fh_dentry returned a DISCONNECTED directory: %s/%s\n",
@@ -295,28 +287,28 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, int type, int access)
 		error = nfsd_set_fh_dentry(rqstp, fhp);
 		if (error)
 			goto out;
-		dentry = fhp->fh_dentry;
-		exp = fhp->fh_export;
-	} else {
-		/*
-		 * just rechecking permissions
-		 * (e.g. nfsproc_create calls fh_verify, then nfsd_create
-		 * does as well)
-		 */
-		dprintk("nfsd: fh_verify - just checking\n");
+	}
 	dentry = fhp->fh_dentry;
 	exp = fhp->fh_export;
 	/*
-		 * Set user creds for this exportpoint; necessary even
-		 * in the "just checking" case because this may be a
-		 * filehandle that was created by fh_compose, and that
-		 * is about to be used in another nfsv4 compound
-		 * operation.
+	 * We still have to do all these permission checks, even when
+	 * fh_dentry is already set:
+	 * 	- fh_verify may be called multiple times with different
+	 * 	  "access" arguments (e.g. nfsd_proc_create calls
+	 * 	  fh_verify(...,NFSD_MAY_EXEC) first, then later (in
+	 * 	  nfsd_create) calls fh_verify(...,NFSD_MAY_CREATE).
+	 *	- in the NFSv4 case, the filehandle may have been filled
+	 *	  in by fh_compose, and given a dentry, but further
+	 *	  compound operations performed with that filehandle
+	 *	  still need permissions checks.  In the worst case, a
+	 *	  mountpoint crossing may have changed the export
+	 *	  options, and we may now need to use a different uid
+	 *	  (for example, if different id-squashing options are in
+	 *	  effect on the new filesystem).
 	 */
 	error = nfsd_setuser_and_check_port(rqstp, exp);
 	if (error)
 		goto out;
-	}
 
 	error = nfsd_mode_check(rqstp, dentry->d_inode->i_mode, type);
 	if (error)