IMA: Add support for file reads without contents

When the kernel_read_file LSM hook is called with contents=false, IMA
can appraise the file directly, without requiring a filled buffer. When
such a buffer is available, though, IMA can continue to use it instead
of forcing a double read here.
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Link: https://lore.kernel.org/lkml/20200706232309.12010-10-scott.branden@broadcom.com/Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/r/20201002173828.2099543-13-keescook@chromium.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

security/integrity/ima/ima_main.c

@@ -613,11 +613,8 @@ void ima_post_path_mknod(struct dentry *dentry)
 int ima_read_file(struct file *file, enum kernel_read_file_id read_id,
 		  bool contents)
 {
-	/* Reject all partial reads during appraisal. */
-	if (!contents) {
-		if (ima_appraise & IMA_APPRAISE_ENFORCE)
-			return -EACCES;
-	}
+	enum ima_hooks func;
+	u32 secid;
 
 	/*
 	 * Do devices using pre-allocated memory run the risk of the
@@ -626,7 +623,20 @@ int ima_read_file(struct file *file, enum kernel_read_file_id read_id,
 	 * buffers? It may be desirable to include the buffer address
 	 * in this API and walk all the dma_map_single() mappings to check.
 	 */
-	return 0;
+
+	/*
+	 * There will be a call made to ima_post_read_file() with
+	 * a filled buffer, so we don't need to perform an extra
+	 * read early here.
+	 */
+	if (contents)
+		return 0;
+
+	/* Read entire file for all partial reads. */
+	func = read_idmap[read_id] ?: FILE_CHECK;
+	security_task_getsecid(current, &secid);
+	return process_measurement(file, current_cred(), secid, NULL,
+				   0, MAY_READ, func);
 }
 
 const int read_idmap[READING_MAX_ID] = {