Introduce SEPARATE_CODE_AND_RODATA build flag
At the moment, all BL images share a similar memory layout: they start
with their code section, followed by their read-only data section.
The two sections are contiguous in memory. Therefore, the end of the
code section and the beginning of the read-only data one might share
a memory page. This forces both to be mapped with the same memory
attributes. As the code needs to be executable, this means that the
read-only data stored on the same memory page as the code are
executable as well. This could potentially be exploited as part of
a security attack.
This patch introduces a new build flag called
SEPARATE_CODE_AND_RODATA, which isolates the code and read-only data
on separate memory pages. This in turn allows independent control of
the access permissions for the code and read-only data.
This has an impact on memory footprint, as padding bytes need to be
introduced between the code and read-only data to ensure the
segragation of the two. To limit the memory cost, the memory layout
of the read-only section has been changed in this case.
- When SEPARATE_CODE_AND_RODATA=0, the layout is unchanged, i.e.
the read-only section still looks like this (padding omitted):
| ... |
+-------------------+
| Exception vectors |
+-------------------+
| Read-only data |
+-------------------+
| Code |
+-------------------+ BLx_BASE
In this case, the linker script provides the limits of the whole
read-only section.
- When SEPARATE_CODE_AND_RODATA=1, the exception vectors and
read-only data are swapped, such that the code and exception
vectors are contiguous, followed by the read-only data. This
gives the following new layout (padding omitted):
| ... |
+-------------------+
| Read-only data |
+-------------------+
| Exception vectors |
+-------------------+
| Code |
+-------------------+ BLx_BASE
In this case, the linker script now exports 2 sets of addresses
instead: the limits of the code and the limits of the read-only
data. Refer to the Firmware Design guide for more details. This
provides platform code with a finer-grained view of the image
layout and allows it to map these 2 regions with the appropriate
access permissions.
Note that SEPARATE_CODE_AND_RODATA applies to all BL images.
Change-Id: I936cf80164f6b66b6ad52b8edacadc532c935a49
diff --git a/Makefile b/Makefile
index 5f4a93c..800312c 100644
--- a/Makefile
+++ b/Makefile
@@ -108,6 +108,10 @@
ENABLE_PMF := 0
# Flag to enable PSCI STATs functionality
ENABLE_PSCI_STAT := 0
+# Whether code and read-only data should be put on separate memory pages.
+# The platform Makefile is free to override this value.
+SEPARATE_CODE_AND_RODATA := 0
+
################################################################################
# Checkpatch script options
@@ -419,6 +423,7 @@
$(eval $(call assert_boolean,PL011_GENERIC_UART))
$(eval $(call assert_boolean,ENABLE_PMF))
$(eval $(call assert_boolean,ENABLE_PSCI_STAT))
+$(eval $(call assert_boolean,SEPARATE_CODE_AND_RODATA))
################################################################################
@@ -448,6 +453,7 @@
$(eval $(call add_define,PL011_GENERIC_UART))
$(eval $(call add_define,ENABLE_PMF))
$(eval $(call add_define,ENABLE_PSCI_STAT))
+$(eval $(call add_define,SEPARATE_CODE_AND_RODATA))
# Define the EL3_PAYLOAD_BASE flag only if it is provided.
ifdef EL3_PAYLOAD_BASE
$(eval $(call add_define,EL3_PAYLOAD_BASE))