knfsd: avoid Oops if buggy userspace performs confusing filehandle->dentry mapping

When a lookup request arrives, nfsd uses information provided by userspace
(mountd) to find the right filesystem.

It then assumes that the same filehandle type as the incoming filehandle can
be used to create an outgoing filehandle.

However if mountd is buggy, or maybe just being creative, the filesystem may
not support that filesystem type, and the kernel could oops, particularly if
'ex_uuid' is NULL but a FSID_UUID* filehandle type is used.

So add some proper checking that the fsid version/type from the incoming
filehandle is actually supportable, and ignore that information if it isn't
supportable.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfsd/nfsfh.c

@@ -323,7 +323,7 @@ fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry,
 	 *
 	 */
 
-	u8 version = 1;
+	u8 version;
 	u8 fsid_type = 0;
 	struct inode * inode = dentry->d_inode;
 	struct dentry *parent = dentry->d_parent;
@@ -341,15 +341,59 @@ fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry,
 	 * the reference filehandle (if it is in the same export)
 	 * or the export options.
 	 */
+ retry:
+	version = 1;
 	if (ref_fh && ref_fh->fh_export == exp) {
 		version = ref_fh->fh_handle.fh_version;
-		if (version == 0xca)
-			fsid_type = FSID_DEV;
-		else
 		fsid_type = ref_fh->fh_handle.fh_fsid_type;
-		/* We know this version/type works for this export
-		 * so there is no need for further checks.
+
+		if (ref_fh == fhp)
+			fh_put(ref_fh);
+		ref_fh = NULL;
+
+		switch (version) {
+		case 0xca:
+			fsid_type = FSID_DEV;
+			break;
+		case 1:
+			break;
+		default:
+			goto retry;
+		}
+
+		/* Need to check that this type works for this
+		 * export point.  As the fsid -> filesystem mapping
+		 * was guided by user-space, there is no guarantee
+		 * that the filesystem actually supports that fsid
+		 * type. If it doesn't we loop around again without
+		 * ref_fh set.
 		 */
+		switch(fsid_type) {
+		case FSID_DEV:
+			if (!old_valid_dev(ex_dev))
+				goto retry;
+			/* FALL THROUGH */
+		case FSID_MAJOR_MINOR:
+		case FSID_ENCODE_DEV:
+			if (!(exp->ex_dentry->d_inode->i_sb->s_type->fs_flags
+			      & FS_REQUIRES_DEV))
+				goto retry;
+			break;
+		case FSID_NUM:
+			if (! (exp->ex_flags & NFSEXP_FSID))
+				goto retry;
+			break;
+		case FSID_UUID8:
+		case FSID_UUID16:
+			if (!root_export)
+				goto retry;
+			/* fall through */
+		case FSID_UUID4_INUM:
+		case FSID_UUID16_INUM:
+			if (exp->ex_uuid == NULL)
+				goto retry;
+			break;
+		}
 	} else if (exp->ex_uuid) {
 		if (fhp->fh_maxsize >= 64) {
 			if (root_export)