[PATCH v3] cryptodisk: allow user to retry failed passphrase

[Forest]

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>
---
 docs/grub.texi              |  9 +++++
 grub-core/disk/cryptodisk.c | 74 +++++++++++++++++++++++++++++--------
 2 files changed, 67 insertions(+), 16 deletions(-)

diff --git a/docs/grub.texi b/docs/grub.texi
index a225f9a88..6ac603a32 100644
--- a/docs/grub.texi
+++ b/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. The default is @samp{3}. (The user can give up early by
+entering an empty passphrase.)
+
+
 @node debug
 @subsection debug
 
diff --git a/grub-core/disk/cryptodisk.c b/grub-core/disk/cryptodisk.c
index 2246af51b..4fa7dc58d 100644
--- a/grub-core/disk/cryptodisk.c
+++ b/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>
@@ -1063,6 +1064,8 @@ grub_cryptodisk_scan_device_real (const char *name,
   grub_cryptodisk_dev_t cr;
   struct cryptodisk_read_hook_ctx read_hook_data = {0};
   int askpass = 0;
+  unsigned long tries = 3;
+  const char *tries_env;
   char *part = NULL;
 
   dev = grub_cryptodisk_get_by_source_disk (source);
@@ -1114,32 +1117,71 @@ 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. */
        askpass = 1;
-       part = grub_partition_get_name (source->partition);
-       grub_printf_ (N_("Enter passphrase for %s%s%s (%s): "), source->name,
-                    source->partition != NULL ? "," : "",
-                    part != NULL ? part : N_("UNKNOWN"),
-                    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))
+       tries_env = grub_env_get ("cryptodisk_passphrase_tries");
+       if (tries_env != NULL && tries_env[0] != '\0')
          {
-           grub_error (GRUB_ERR_BAD_ARGUMENT, "passphrase not supplied");
-           goto error;
+           const char *p = NULL;
+           tries = grub_strtoul (tries_env, &p, 0);
+           if (*p != '\0')
+             {
+               /* Account for grub_strtoul() ignoring trailing text. */
+               grub_err_t err = grub_errno;
+               if (p > tries_env && tries != ~0UL)
+                 err = GRUB_ERR_BAD_NUMBER;
+
+               grub_error (err,
+                           N_("non-numeric or invalid value for 
cryptodisk_passphrase_tries: `%s'"),
+                           tries_env);
+               goto error;
+             }
          }
-       cargs->key_len = grub_strlen ((char *) cargs->key_data);
-      }
 
-    ret = cr->recover_key (source, dev, cargs);
-    if (ret != GRUB_ERR_NONE)
-      goto error;
+       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 ? "," : "",
+                        part != NULL ? part : N_("UNKNOWN"),
+                        dev->uuid);
+           grub_free (part);
+
+           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)
+             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)
-- 
2.39.2