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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
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