NFS: Prevent memory allocation failure in nfsacl_encode()

nfsacl_encode() allocates memory in certain cases.  This of course
is not guaranteed to work.

Since commit 9f06c719 "SUNRPC: New xdr_streams XDR encoder API", the
kernel's XDR encoders can't return a result indicating possibly a
failure, so a memory allocation failure in nfsacl_encode() has become
fatal (ie, the XDR code Oopses) in some cases.

However, the allocated memory is a tiny fixed amount, on the order
of 40-50 bytes.  We can easily use a stack-allocated buffer for
this, with only a wee bit of nose-holding.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs3acl.c

@@ -311,8 +311,8 @@ static int nfs3_proc_setacls(struct inode *inode, struct posix_acl *acl,
 	if (!nfs_server_capable(inode, NFS_CAP_ACLS))
 		goto out;
 
-	/* We are doing this here, because XDR marshalling can only
-	   return -ENOMEM. */
+	/* We are doing this here because XDR marshalling does not
+	 * return any results, it BUGs. */
 	status = -ENOSPC;
 	if (acl != NULL && acl->a_count > NFS_ACL_MAX_ENTRIES)
 		goto out;

fs/nfs_common/nfsacl.c

@@ -42,6 +42,11 @@ struct nfsacl_encode_desc {
 	gid_t gid;
 };
 
+struct nfsacl_simple_acl {
+	struct posix_acl acl;
+	struct posix_acl_entry ace[4];
+};
+
 static int
 xdr_nfsace_encode(struct xdr_array2_desc *desc, void *elem)
 {
@@ -99,17 +104,22 @@ int nfsacl_encode(struct xdr_buf *buf, unsigned int base, struct inode *inode,
 		.uid = inode->i_uid,
 		.gid = inode->i_gid,
 	};
+	struct nfsacl_simple_acl aclbuf;
 	int err;
-	struct posix_acl *acl2 = NULL;
 
 	if (entries > NFS_ACL_MAX_ENTRIES ||
 	    xdr_encode_word(buf, base, entries))
 		return -EINVAL;
 	if (encode_entries && acl && acl->a_count == 3) {
-		/* Fake up an ACL_MASK entry. */
-		acl2 = posix_acl_alloc(4, GFP_KERNEL);
-		if (!acl2)
-			return -ENOMEM;
+		struct posix_acl *acl2 = &aclbuf.acl;
+
+		/* Avoid the use of posix_acl_alloc().  nfsacl_encode() is
+		 * invoked in contexts where a memory allocation failure is
+		 * fatal.  Fortunately this fake ACL is small enough to
+		 * construct on the stack. */
+		memset(acl2, 0, sizeof(acl2));
+		posix_acl_init(acl2, 4);
+
 		/* Insert entries in canonical order: other orders seem
 		 to confuse Solaris VxFS. */
 		acl2->a_entries[0] = acl->a_entries[0];  /* ACL_USER_OBJ */
@@ -120,8 +130,6 @@ int nfsacl_encode(struct xdr_buf *buf, unsigned int base, struct inode *inode,
 		nfsacl_desc.acl = acl2;
 	}
 	err = xdr_encode_array2(buf, base + 4, &nfsacl_desc.desc);
-	if (acl2)
-		posix_acl_release(acl2);
 	if (!err)
 		err = 8 + nfsacl_desc.desc.elem_size *
 			  nfsacl_desc.desc.array_len;

fs/posix_acl.c

@@ -22,6 +22,7 @@
 
 #include <linux/errno.h>
 
+EXPORT_SYMBOL(posix_acl_init);
 EXPORT_SYMBOL(posix_acl_alloc);
 EXPORT_SYMBOL(posix_acl_clone);
 EXPORT_SYMBOL(posix_acl_valid);
@@ -31,6 +32,16 @@ EXPORT_SYMBOL(posix_acl_create_masq);
 EXPORT_SYMBOL(posix_acl_chmod_masq);
 EXPORT_SYMBOL(posix_acl_permission);
 
+/*
+ * Init a fresh posix_acl
+ */
+void
+posix_acl_init(struct posix_acl *acl, int count)
+{
+	atomic_set(&acl->a_refcount, 1);
+	acl->a_count = count;
+}
+
 /*
  * Allocate a new ACL with the specified number of entries.
  */
@@ -40,10 +51,8 @@ posix_acl_alloc(int count, gfp_t flags)
 	const size_t size = sizeof(struct posix_acl) +
 	                    count * sizeof(struct posix_acl_entry);
 	struct posix_acl *acl = kmalloc(size, flags);
-	if (acl) {
-		atomic_set(&acl->a_refcount, 1);
-		acl->a_count = count;
-	}
+	if (acl)
+		posix_acl_init(acl, count);
 	return acl;
 }
 

include/linux/posix_acl.h

@@ -71,6 +71,7 @@ posix_acl_release(struct posix_acl *acl)
 
 /* posix_acl.c */
 
+extern void posix_acl_init(struct posix_acl *, int);
 extern struct posix_acl *posix_acl_alloc(int, gfp_t);
 extern struct posix_acl *posix_acl_clone(const struct posix_acl *, gfp_t);
 extern int posix_acl_valid(const struct posix_acl *);