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

Makefile

@@ -108,6 +108,10 @@ PL011_GENERIC_UART		:= 0
 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,SPIN_ON_BL1_EXIT))
 $(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,SPIN_ON_BL1_EXIT))
 $(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))

bl1/bl1.ld.S

@@ -45,6 +45,43 @@ SECTIONS
     ASSERT(. == ALIGN(4096),
            "BL1_RO_BASE address is not aligned on a page boundary.")
 
+#if SEPARATE_CODE_AND_RODATA
+    .text . : {
+        __TEXT_START__ = .;
+        *bl1_entrypoint.o(.text*)
+        *(.text*)
+        *(.vectors)
+        . = NEXT(4096);
+        __TEXT_END__ = .;
+     } >ROM
+
+    .rodata . : {
+        __RODATA_START__ = .;
+        *(.rodata*)
+
+        /* Ensure 8-byte alignment for descriptors and ensure inclusion */
+        . = ALIGN(8);
+        __PARSER_LIB_DESCS_START__ = .;
+        KEEP(*(.img_parser_lib_descs))
+        __PARSER_LIB_DESCS_END__ = .;
+
+        /*
+         * Ensure 8-byte alignment for cpu_ops so that its fields are also
+         * aligned. Also ensure cpu_ops inclusion.
+         */
+        . = ALIGN(8);
+        __CPU_OPS_START__ = .;
+        KEEP(*(cpu_ops))
+        __CPU_OPS_END__ = .;
+
+        /*
+         * No need to pad out the .rodata section to a page boundary. Next is
+         * the .data section, which can mapped in ROM with the same memory
+         * attributes as the .rodata section.
+         */
+        __RODATA_END__ = .;
+    } >ROM
+#else
     ro . : {
         __RO_START__ = .;
         *bl1_entrypoint.o(.text*)
@@ -69,6 +106,7 @@ SECTIONS
         *(.vectors)
         __RO_END__ = .;
     } >ROM
+#endif
 
     ASSERT(__CPU_OPS_END__ > __CPU_OPS_START__,
            "cpu_ops not defined for this platform.")

bl2/bl2.ld.S

@@ -45,6 +45,30 @@ SECTIONS
     ASSERT(. == ALIGN(4096),
            "BL2_BASE address is not aligned on a page boundary.")
 
+#if SEPARATE_CODE_AND_RODATA
+    .text . : {
+        __TEXT_START__ = .;
+        *bl2_entrypoint.o(.text*)
+        *(.text*)
+        *(.vectors)
+        . = NEXT(4096);
+        __TEXT_END__ = .;
+     } >RAM
+
+    .rodata . : {
+        __RODATA_START__ = .;
+        *(.rodata*)
+
+        /* Ensure 8-byte alignment for descriptors and ensure inclusion */
+        . = ALIGN(8);
+        __PARSER_LIB_DESCS_START__ = .;
+        KEEP(*(.img_parser_lib_descs))
+        __PARSER_LIB_DESCS_END__ = .;
+
+        . = NEXT(4096);
+        __RODATA_END__ = .;
+    } >RAM
+#else
     ro . : {
         __RO_START__ = .;
         *bl2_entrypoint.o(.text*)
@@ -67,6 +91,7 @@ SECTIONS
         . = NEXT(4096);
         __RO_END__ = .;
     } >RAM
+#endif
 
     /*
      * Define a linker symbol to mark start of the RW memory area for this

bl2u/bl2u.ld.S

@@ -45,6 +45,23 @@ SECTIONS
     ASSERT(. == ALIGN(4096),
            "BL2U_BASE address is not aligned on a page boundary.")
 
+#if SEPARATE_CODE_AND_RODATA
+    .text . : {
+        __TEXT_START__ = .;
+        *bl2u_entrypoint.o(.text*)
+        *(.text*)
+        *(.vectors)
+        . = NEXT(4096);
+        __TEXT_END__ = .;
+     } >RAM
+
+    .rodata . : {
+        __RODATA_START__ = .;
+        *(.rodata*)
+        . = NEXT(4096);
+        __RODATA_END__ = .;
+    } >RAM
+#else
     ro . : {
         __RO_START__ = .;
         *bl2u_entrypoint.o(.text*)
@@ -61,6 +78,7 @@ SECTIONS
         . = NEXT(4096);
         __RO_END__ = .;
     } >RAM
+#endif
 
     /*
      * Define a linker symbol to mark start of the RW memory area for this

bl31/bl31.ld.S

@@ -46,6 +46,47 @@ SECTIONS
     ASSERT(. == ALIGN(4096),
            "BL31_BASE address is not aligned on a page boundary.")
 
+#if SEPARATE_CODE_AND_RODATA
+    .text . : {
+        __TEXT_START__ = .;
+        *bl31_entrypoint.o(.text*)
+        *(.text*)
+        *(.vectors)
+        . = NEXT(4096);
+        __TEXT_END__ = .;
+    } >RAM
+
+    .rodata . : {
+        __RODATA_START__ = .;
+        *(.rodata*)
+
+        /* Ensure 8-byte alignment for descriptors and ensure inclusion */
+        . = ALIGN(8);
+        __RT_SVC_DESCS_START__ = .;
+        KEEP(*(rt_svc_descs))
+        __RT_SVC_DESCS_END__ = .;
+
+#if ENABLE_PMF
+        /* Ensure 8-byte alignment for descriptors and ensure inclusion */
+        . = ALIGN(8);
+        __PMF_SVC_DESCS_START__ = .;
+        KEEP(*(pmf_svc_descs))
+        __PMF_SVC_DESCS_END__ = .;
+#endif /* ENABLE_PMF */
+
+        /*
+         * Ensure 8-byte alignment for cpu_ops so that its fields are also
+         * aligned. Also ensure cpu_ops inclusion.
+         */
+        . = ALIGN(8);
+        __CPU_OPS_START__ = .;
+        KEEP(*(cpu_ops))
+        __CPU_OPS_END__ = .;
+
+        . = NEXT(4096);
+        __RODATA_END__ = .;
+    } >RAM
+#else
     ro . : {
         __RO_START__ = .;
         *bl31_entrypoint.o(.text*)
@@ -85,6 +126,7 @@ SECTIONS
         . = NEXT(4096);
         __RO_END__ = .;
     } >RAM
+#endif
 
     ASSERT(__CPU_OPS_END__ > __CPU_OPS_START__,
            "cpu_ops not defined for this platform.")

bl32/tsp/tsp.ld.S

@@ -46,6 +46,23 @@ SECTIONS
     ASSERT(. == ALIGN(4096),
            "BL32_BASE address is not aligned on a page boundary.")
 
+#if SEPARATE_CODE_AND_RODATA
+    .text . : {
+        __TEXT_START__ = .;
+        *tsp_entrypoint.o(.text*)
+        *(.text*)
+        *(.vectors)
+        . = NEXT(4096);
+        __TEXT_END__ = .;
+    } >RAM
+
+    .rodata . : {
+        __RODATA_START__ = .;
+        *(.rodata*)
+        . = NEXT(4096);
+        __RODATA_END__ = .;
+    } >RAM
+#else
     ro . : {
         __RO_START__ = .;
         *tsp_entrypoint.o(.text*)
@@ -61,6 +78,7 @@ SECTIONS
         . = NEXT(4096);
         __RO_END__ = .;
     } >RAM
+#endif
 
     /*
      * Define a linker symbol to mark start of the RW memory area for this

docs/firmware-design.md

@@ -1115,7 +1115,9 @@ All BL images share the following requirements:
 *   The BSS section must be zero-initialised before executing any C code.
 *   The coherent memory section (if enabled) must be zero-initialised as well.
 *   The MMU setup code needs to know the extents of the coherent and read-only
-    memory regions to set the right memory attributes.
+    memory regions to set the right memory attributes. When
+    `SEPARATE_CODE_AND_RODATA=1`, it needs to know more specifically how the
+    read-only memory region is divided between code and data.
 
 The following linker symbols are defined for this purpose:
 
@@ -1126,6 +1128,10 @@ The following linker symbols are defined for this purpose:
 *   `__COHERENT_RAM_UNALIGNED_SIZE__`
 *   `__RO_START__`
 *   `__RO_END__`
+*   `__TEXT_START__`
+*   `__TEXT_END__`
+*   `__RODATA_START__`
+*   `__RODATA_END__`
 
 #### BL1's linker symbols
 

include/common/bl_common.h

@@ -143,8 +143,16 @@
  * Declarations of linker defined symbols to help determine memory layout of
  * BL images
  */
+#if SEPARATE_CODE_AND_RODATA
+extern unsigned long __TEXT_START__;
+extern unsigned long __TEXT_END__;
+extern unsigned long __RODATA_START__;
+extern unsigned long __RODATA_END__;
+#else
 extern unsigned long __RO_START__;
 extern unsigned long __RO_END__;
+#endif
+
 #if IMAGE_BL2
 extern unsigned long __BL2_END__;
 #elif IMAGE_BL2U