refactor(console): disable getc() by default
The ability to read a character from the console constitutes an attack
vector into TF-A, as it gives attackers a means to inject arbitrary
data into TF-A. It is dangerous to keep that feature enabled if not
strictly necessary, especially in production firmware builds.
Thus, we need a way to disable this feature. Moreover, when it is
disabled, all related code should be eliminated from the firmware
binaries, such that no remnant/dead getc() code remains in memory,
which could otherwise be used as a gadget as part of a bigger security
attack.
This patch disables getc() feature by default. For legitimate getc()
use cases [1], it can be explicitly enabled by building TF-A with
ENABLE_CONSOLE_GETC=1.
The following changes are introduced when getc() is disabled:
- The multi-console framework no longer provides the console_getc()
function.
- If the console driver selected by the platform attempts to register
a getc() callback into the multi-console framework then TF-A will
now fail to build.
If registered through the assembly function finish_console_register():
- On AArch64, you'll get:
Error: undefined symbol CONSOLE_T_GETC used as an immediate value.
- On AArch32, you'll get:
Error: internal_relocation (type: OFFSET_IMM) not fixed up
If registered through the C function console_register(), this requires
populating a struct console with a getc field, which will trigger:
error: 'console_t' {aka 'struct console'} has no member named 'getc'
- All console drivers which previously registered a getc() callback
have been modified to do so only when ENABLE_CONSOLE_GETC=1.
[1] Example of such use cases would be:
- Firmware recovery: retrieving a golden BL2 image over the console in
order to repair a broken firmware on a bricked board.
- Factory CLI tool: Drive some soak tests through the console.
Discussed on TF-A mailing list here:
https://lists.trustedfirmware.org/archives/list/tf-a@lists.trustedfirmware.org/thread/YS7F6RCNTWBTEOBLAXIRTXWIOYINVRW7/
Change-Id: Icb412304cd23dbdd7662df7cf8992267b7975cc5
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
diff --git a/include/arch/aarch64/console_macros.S b/include/arch/aarch64/console_macros.S
index 3285d85..8adb9cd 100644
--- a/include/arch/aarch64/console_macros.S
+++ b/include/arch/aarch64/console_macros.S
@@ -30,12 +30,19 @@
str xzr, [x0, #CONSOLE_T_PUTC]
.endif
+ /*
+ * If ENABLE_CONSOLE_GETC support is disabled, but a getc callback is
+ * specified nonetheless, the assembler will abort on encountering the
+ * CONSOLE_T_GETC macro, which is undefined.
+ */
.ifne \getc
adrp x1, console_\_driver\()_getc
add x1, x1, :lo12:console_\_driver\()_getc
str x1, [x0, #CONSOLE_T_GETC]
.else
+#if ENABLE_CONSOLE_GETC
str xzr, [x0, #CONSOLE_T_GETC]
+#endif
.endif
.ifne \flush