fs/kernel_file_read: Add "offset" arg for partial reads

To perform partial reads, callers of kernel_read_file*() must have a
non-NULL file_size argument and a preallocated buffer. The new "offset"
argument can then be used to seek to specific locations in the file to
fill the buffer to, at most, "buf_size" per call.

Where possible, the LSM hooks can report whether a full file has been
read or not so that the contents can be reasoned about.

Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/firmware_loader/main.c

···
 		fw_priv->size = 0;
 
 		/* load firmware files from the mount namespace of init */
-		rc = kernel_read_file_from_path_initns(path, &buffer, msize,
+		rc = kernel_read_file_from_path_initns(path, 0, &buffer, msize,
 						       NULL,
 						       READING_FIRMWARE);
 		if (rc < 0) {

fs/kernel_read_file.c

···
  * kernel_read_file() - read file contents into a kernel buffer
  *
  * @file	file to read from
+ * @offset	where to start reading from (see below).
  * @buf		pointer to a "void *" buffer for reading into (if
  *		*@buf is NULL, a buffer will be allocated, and
  *		@buf_size will be ignored)
···
  * @id		the kernel_read_file_id identifying the type of
  *		file contents being read (for LSMs to examine)
  *
+ * @offset must be 0 unless both @buf and @file_size are non-NULL
+ * (i.e. the caller must be expecting to read partial file contents
+ * via an already-allocated @buf, in at most @buf_size chunks, and
+ * will be able to determine when the entire file was read by
+ * checking @file_size). This isn't a recommended way to read a
+ * file, though, since it is possible that the contents might
+ * change between calls to kernel_read_file().
+ *
  * Returns number of bytes read (no single read will be bigger
  * than INT_MAX), or negative on error.
  *
  */
-int kernel_read_file(struct file *file, void **buf,
+int kernel_read_file(struct file *file, loff_t offset, void **buf,
 		     size_t buf_size, size_t *file_size,
 		     enum kernel_read_file_id id)
 {
 	loff_t i_size, pos;
-	ssize_t bytes = 0;
+	size_t copied;
 	void *allocated = NULL;
+	bool whole_file;
 	int ret;
+
+	if (offset != 0 && (!*buf || !file_size))
+		return -EINVAL;
 
 	if (!S_ISREG(file_inode(file)->i_mode))
 		return -EINVAL;
···
 	if (ret)
 		return ret;
 
-	ret = security_kernel_read_file(file, id, true);
-	if (ret)
-		goto out;
-
 	i_size = i_size_read(file_inode(file));
 	if (i_size <= 0) {
 		ret = -EINVAL;
 		goto out;
 	}
-	if (i_size > INT_MAX || i_size > buf_size) {
+	/* The file is too big for sane activities. */
+	if (i_size > INT_MAX) {
 		ret = -EFBIG;
 		goto out;
 	}
+	/* The entire file cannot be read in one buffer. */
+	if (!file_size && offset == 0 && i_size > buf_size) {
+		ret = -EFBIG;
+		goto out;
+	}
+
+	whole_file = (offset == 0 && i_size <= buf_size);
+	ret = security_kernel_read_file(file, id, whole_file);
+	if (ret)
+		goto out;
+
 	if (file_size)
 		*file_size = i_size;
 
···
 		goto out;
 	}
 
-	pos = 0;
-	while (pos < i_size) {
-		bytes = kernel_read(file, *buf + pos, i_size - pos, &pos);
+	pos = offset;
+	copied = 0;
+	while (copied < buf_size) {
+		ssize_t bytes;
+		size_t wanted = min_t(size_t, buf_size - copied,
+					      i_size - pos);
+
+		bytes = kernel_read(file, *buf + copied, wanted, &pos);
 		if (bytes < 0) {
 			ret = bytes;
 			goto out_free;
···
 
 		if (bytes == 0)
 			break;
+		copied += bytes;
 	}
 
-	if (pos != i_size) {
-		ret = -EIO;
-		goto out_free;
-	}
+	if (whole_file) {
+		if (pos != i_size) {
+			ret = -EIO;
+			goto out_free;
+		}
 
-	ret = security_kernel_post_read_file(file, *buf, i_size, id);
+		ret = security_kernel_post_read_file(file, *buf, i_size, id);
+	}
 
 out_free:
 	if (ret < 0) {
···
 
 out:
 	allow_write_access(file);
-	return ret == 0 ? pos : ret;
+	return ret == 0 ? copied : ret;
 }
 EXPORT_SYMBOL_GPL(kernel_read_file);
 
-int kernel_read_file_from_path(const char *path, void **buf,
+int kernel_read_file_from_path(const char *path, loff_t offset, void **buf,
 			       size_t buf_size, size_t *file_size,
 			       enum kernel_read_file_id id)
 {
···
 	if (IS_ERR(file))
 		return PTR_ERR(file);
 
-	ret = kernel_read_file(file, buf, buf_size, file_size, id);
+	ret = kernel_read_file(file, offset, buf, buf_size, file_size, id);
 	fput(file);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(kernel_read_file_from_path);
 
-int kernel_read_file_from_path_initns(const char *path, void **buf,
-				      size_t buf_size, size_t *file_size,
+int kernel_read_file_from_path_initns(const char *path, loff_t offset,
+				      void **buf, size_t buf_size,
+				      size_t *file_size,
 				      enum kernel_read_file_id id)
 {
 	struct file *file;
···
 	if (IS_ERR(file))
 		return PTR_ERR(file);
 
-	ret = kernel_read_file(file, buf, buf_size, file_size, id);
+	ret = kernel_read_file(file, offset, buf, buf_size, file_size, id);
 	fput(file);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(kernel_read_file_from_path_initns);
 
-int kernel_read_file_from_fd(int fd, void **buf, size_t buf_size,
-			     size_t *file_size,
+int kernel_read_file_from_fd(int fd, loff_t offset, void **buf,
+			     size_t buf_size, size_t *file_size,
 			     enum kernel_read_file_id id)
 {
 	struct fd f = fdget(fd);
···
 	if (!f.file)
 		goto out;
 
-	ret = kernel_read_file(f.file, buf, buf_size, file_size, id);
+	ret = kernel_read_file(f.file, offset, buf, buf_size, file_size, id);
 out:
 	fdput(f);
 	return ret;

include/linux/kernel_read_file.h

···
 	return kernel_read_file_str[id];
 }
 
-int kernel_read_file(struct file *file,
+int kernel_read_file(struct file *file, loff_t offset,
 		     void **buf, size_t buf_size,
 		     size_t *file_size,
 		     enum kernel_read_file_id id);
-int kernel_read_file_from_path(const char *path,
+int kernel_read_file_from_path(const char *path, loff_t offset,
 			       void **buf, size_t buf_size,
 			       size_t *file_size,
 			       enum kernel_read_file_id id);
-int kernel_read_file_from_path_initns(const char *path,
+int kernel_read_file_from_path_initns(const char *path, loff_t offset,
 				      void **buf, size_t buf_size,
 				      size_t *file_size,
 				      enum kernel_read_file_id id);
-int kernel_read_file_from_fd(int fd,
+int kernel_read_file_from_fd(int fd, loff_t offset,
 			     void **buf, size_t buf_size,
 			     size_t *file_size,
 			     enum kernel_read_file_id id);

kernel/kexec_file.c

···
 	int ret;
 	void *ldata;
 
-	ret = kernel_read_file_from_fd(kernel_fd, &image->kernel_buf,
+	ret = kernel_read_file_from_fd(kernel_fd, 0, &image->kernel_buf,
 				       INT_MAX, NULL, READING_KEXEC_IMAGE);
 	if (ret < 0)
 		return ret;
···
 #endif
 	/* It is possible that there no initramfs is being loaded */
 	if (!(flags & KEXEC_FILE_NO_INITRAMFS)) {
-		ret = kernel_read_file_from_fd(initrd_fd, &image->initrd_buf,
+		ret = kernel_read_file_from_fd(initrd_fd, 0, &image->initrd_buf,
 					       INT_MAX, NULL,
 					       READING_KEXEC_INITRAMFS);
 		if (ret < 0)

kernel/module.c

···
 		      |MODULE_INIT_IGNORE_VERMAGIC))
 		return -EINVAL;
 
-	err = kernel_read_file_from_fd(fd, &hdr, INT_MAX, NULL,
+	err = kernel_read_file_from_fd(fd, 0, &hdr, INT_MAX, NULL,
 				       READING_MODULE);
 	if (err < 0)
 		return err;

security/integrity/digsig.c

···
 	int rc;
 	key_perm_t perm;
 
-	rc = kernel_read_file_from_path(path, &data, INT_MAX, NULL,
+	rc = kernel_read_file_from_path(path, 0, &data, INT_MAX, NULL,
 					READING_X509_CERTIFICATE);
 	if (rc < 0) {
 		pr_err("Unable to open file: %s (%d)", path, rc);

security/integrity/ima/ima_fs.c

···
 	datap = path;
 	strsep(&datap, "\n");
 
-	rc = kernel_read_file_from_path(path, &data, INT_MAX, NULL, READING_POLICY);
+	rc = kernel_read_file_from_path(path, 0, &data, INT_MAX, NULL,
+					READING_POLICY);
 	if (rc < 0) {
 		pr_err("Unable to open file: %s (%d)", path, rc);
 		return rc;