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Introduce SEPARATE_CODE_AND_RODATA build flag

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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
This commit is contained in:
Sandrine Bailleux 2016-07-08 14:37:40 +01:00
parent 0146ae64c0
commit 5d1c104f9a
8 changed files with 162 additions and 1 deletions

6
Makefile
View file

@ -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))

38
bl1/bl1.ld.S
View file

@ -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.")

25
bl2/bl2.ld.S
View file

@ -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

18
bl2u/bl2u.ld.S
View file

@ -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

42
bl31/bl31.ld.S
View file

@ -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.")

18
bl32/tsp/tsp.ld.S
View file

@ -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

8
docs/firmware-design.md
View file

@ -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

8
include/common/bl_common.h
View file

@ -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