Read-only xlat tables for BL31 memory
This patch introduces a build flag which allows the xlat tables
to be mapped in a read-only region within BL31 memory. It makes it
much harder for someone who has acquired the ability to write to
arbitrary secure memory addresses to gain control of the
translation tables.
The memory attributes of the descriptors describing the tables
themselves are changed to read-only secure data. This change
happens at the end of BL31 runtime setup. Until this point, the
tables have read-write permissions. This gives a window of
opportunity for changes to be made to the tables with the MMU on
(e.g. reclaiming init code). No changes can be made to the tables
with the MMU turned on from this point onwards. This change is also
enabled for sp_min and tspd.
To make all this possible, the base table was moved to .rodata. The
penalty we pay is that now .rodata must be aligned to the size of
the base table (512B alignment). Still, this is better than putting
the base table with the higher level tables in the xlat_table
section, as that would cost us a full 4KB page.
Changing the tables from read-write to read-only cannot be done with
the MMU on, as the break-before-make sequence would invalidate the
descriptor which resolves the level 3 page table where that very
descriptor is located. This would make the translation required for
writing the changes impossible, generating an MMU fault.
The caches are also flushed.
Signed-off-by: Petre-Ionut Tudor <petre-ionut.tudor@arm.com>
Change-Id: Ibe5de307e6dc94c67d6186139ac3973516430466
diff --git a/plat/arm/common/arm_common.c b/plat/arm/common/arm_common.c
index d1e9620..d1eee08 100644
--- a/plat/arm/common/arm_common.c
+++ b/plat/arm/common/arm_common.c
@@ -25,6 +25,26 @@
* conflicts with the definition in plat/common. */
#pragma weak plat_get_syscnt_freq2
+/*******************************************************************************
+ * Changes the memory attributes for the region of mapped memory where the BL
+ * image's translation tables are located such that the tables will have
+ * read-only permissions.
+ ******************************************************************************/
+#if PLAT_RO_XLAT_TABLES
+void arm_xlat_make_tables_readonly(void)
+{
+ int rc = xlat_make_tables_readonly();
+
+ if (rc != 0) {
+ ERROR("Failed to make translation tables read-only at EL%u.\n",
+ get_current_el());
+ panic();
+ }
+
+ INFO("Translation tables are now read-only at EL%u.\n",
+ get_current_el());
+}
+#endif
void arm_setup_romlib(void)
{