disk/cryptodisk: Allow user to retry failed passphrase

Give the user a chance to re-enter their cryptodisk passphrase after a typo,
rather than immediately failing (and likely dumping them into a GRUB shell).

By default, we allow 3 tries before giving up. A value in the
cryptodisk_passphrase_tries environment variable will override this default.

The user can give up early by entering an empty passphrase, just as they
could before this patch.

Signed-off-by: Forest <forestix@nom.one>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

docs/grub.texi

@@ -3277,6 +3277,7 @@ These variables have special meaning to GRUB.
 * color_normal::
 * config_directory::
 * config_file::
+* cryptodisk_passphrase_tries::
 * debug::
 * default::
 * fallback::
@@ -3441,6 +3442,14 @@ processed by commands @command{configfile} (@pxref{configfile}) or @command{norm
 (@pxref{normal}).  It is restored to the previous value when command completes.
 
 
+@node cryptodisk_passphrase_tries
+@subsection cryptodisk_passphrase_tries
+
+When prompting the user for a cryptodisk passphrase, allow this many attempts
+before giving up. Defaults to @samp{3} if unset or set to an invalid value.
+(The user can give up early by entering an empty passphrase.)
+
+
 @node debug
 @subsection debug
 

grub-core/disk/cryptodisk.c

@@ -17,6 +17,7 @@
  */
 
 #include <grub/cryptodisk.h>
+#include <grub/env.h>
 #include <grub/mm.h>
 #include <grub/misc.h>
 #include <grub/dl.h>
@@ -1114,10 +1115,40 @@ grub_cryptodisk_scan_device_real (const char *name,
     if (!dev)
       continue;
 
-    if (!cargs->key_len)
+    if (cargs->key_len)
+      {
+	ret = cr->recover_key (source, dev, cargs);
+	if (ret != GRUB_ERR_NONE)
+	  goto error;
+      }
+    else
       {
 	/* Get the passphrase from the user, if no key data. */
+	unsigned long tries = 3;
+	const char *tries_env;
+
 	askpass = 1;
+	cargs->key_data = grub_malloc (GRUB_CRYPTODISK_MAX_PASSPHRASE);
+	if (cargs->key_data == NULL)
+	  goto error_no_close;
+
+	tries_env = grub_env_get ("cryptodisk_passphrase_tries");
+	if (tries_env != NULL && tries_env[0] != '\0')
+	  {
+	    unsigned long tries_env_val;
+	    const char *p;
+
+	    tries_env_val = grub_strtoul (tries_env, &p, 0);
+	    if (*p == '\0' && tries_env_val != ~0UL)
+	      tries = tries_env_val;
+	    else
+	      grub_printf_ (N_("Invalid cryptodisk_passphrase_tries value `%s'. Defaulting to %lu.\n"),
+			    tries_env,
+			    tries);
+	  }
+
+	for (; tries > 0; tries--)
+	  {
 	    part = grub_partition_get_name (source->partition);
 	    grub_printf_ (N_("Enter passphrase for %s%s%s (%s): "), source->name,
 			 source->partition != NULL ? "," : "",
@@ -1125,21 +1156,29 @@ grub_cryptodisk_scan_device_real (const char *name,
 			 dev->uuid);
 	    grub_free (part);
 
-	cargs->key_data = grub_malloc (GRUB_CRYPTODISK_MAX_PASSPHRASE);
-	if (cargs->key_data == NULL)
-	  goto error_no_close;
-
 	    if (!grub_password_get ((char *) cargs->key_data, GRUB_CRYPTODISK_MAX_PASSPHRASE))
 	      {
 		grub_error (GRUB_ERR_BAD_ARGUMENT, "passphrase not supplied");
 		goto error;
 	      }
 	    cargs->key_len = grub_strlen ((char *) cargs->key_data);
-      }
 
 	    ret = cr->recover_key (source, dev, cargs);
-    if (ret != GRUB_ERR_NONE)
+	    if (ret == GRUB_ERR_NONE)
+	      break;
+	    if (ret != GRUB_ERR_ACCESS_DENIED || tries == 1)
 	      goto error;
+	    grub_puts_ (N_("Invalid passphrase."));
+
+	    /*
+	     * Since recover_key() calls a function that returns grub_errno,
+	     * a leftover error value from a previously rejected passphrase
+	     * will trigger a phantom failure. We therefore clear it before
+	     * trying a new passphrase.
+	     */
+	    grub_errno = GRUB_ERR_NONE;
+	  }
+      }
 
     ret = grub_cryptodisk_insert (dev, name, source);
     if (ret != GRUB_ERR_NONE)