[PATCH] efivars: Sanitise string length returned byGetNextVariableName()

[Matt Fleming]

Some buggy firmware implementations return a string length from
GetNextVariableName() that is actually larger than the string in
'variable_name', as Michael Schroeder writes,

  > On HP z220 system (firmware version 1.54), some EFI variables are
  > incorrectly named :
  >
  > ls -d /sys/firmware/efi/vars/*8be4d* | grep -v -- -8be returns
  > /sys/firmware/efi/vars/dbxDefault-pport8be4df61-93ca-11d2-aa0d-00e098032b8c
  > /sys/firmware/efi/vars/KEKDefault-pport8be4df61-93ca-11d2-aa0d-00e098032b8c
  > /sys/firmware/efi/vars/SecureBoot-pport8be4df61-93ca-11d2-aa0d-00e098032b8c
  > /sys/firmware/efi/vars/SetupMode-Information8be4df61-93ca-11d2-aa0d-00e098032b8c

Since 'variable_name' is a string, we can validate its size by
searching for the terminating NULL character.

Reported-by: Frederic Crozat <fcrozat@xxxxxxxx>
Cc: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
Cc: Josh Boyer <jwboyer@xxxxxxxxxx>
Cc: Michael Schroeder <mls@xxxxxxxx>
Cc: Lee, Chun-Yi <jlee@xxxxxxxx>
Cc: Lingzhu Xiang <lxiang@xxxxxxxxxx>
Signed-off-by: Matt Fleming <matt.fleming@xxxxxxxxx>
---
 drivers/firmware/efivars.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efivars.c b/drivers/firmware/efivars.c
index 7320bf8..b5af292 100644
--- a/drivers/firmware/efivars.c
+++ b/drivers/firmware/efivars.c
@@ -1653,6 +1653,33 @@ static bool variable_is_present(efi_char16_t *variable_name, efi_guid_t *vendor)
 	return found;
 }
 
+/*
+ * Sanity check string length of a variable name.
+ */
+static unsigned long sanity_check_strlen(efi_char16_t *variable_name,
+					 unsigned long variable_name_size)
+{
+	efi_char16_t c;
+	unsigned long len;
+
+	/*
+	 * The variable name is, by definition, a NULL-terminated
+	 * string, so make absolutely sure that variable_name_size is
+	 * the value we expect it to be. If not, use whichever size is
+	 * smaller.
+	 */
+	for (len = 1; len <= variable_name_size; len++) {
+		c = variable_name[len - 1];
+		if (!c)
+			break;
+	}
+
+	if (len != variable_name_size)
+		printk(KERN_WARNING "efivars: bogus variable_name_size\n");
+
+	return min(len, variable_name_size);
+}
+
 static void efivar_update_sysfs_entries(struct work_struct *work)
 {
 	struct efivars *efivars = &__efivars;
@@ -1693,10 +1720,13 @@ static void efivar_update_sysfs_entries(struct work_struct *work)
 		if (!found) {
 			kfree(variable_name);
 			break;
-		} else
+		} else {
+			variable_name_size = sanity_check_strlen(variable_name,
+								 variable_name_size);
 			efivar_create_sysfs_entry(efivars,
 						  variable_name_size,
 						  variable_name, &vendor);
+		}
 	}
 }
 
@@ -1941,8 +1971,11 @@ int register_efivars(struct efivars *efivars,
 		status = ops->get_next_variable(&variable_name_size,
 						variable_name,
 						&vendor_guid);
+
 		switch (status) {
 		case EFI_SUCCESS:
+			variable_name_size = sanity_check_strlen(variable_name,
+								 variable_name_size);
 			efivar_create_sysfs_entry(efivars,
 						  variable_name_size,
 						  variable_name,
-- 
1.7.11.7


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