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/drivers/console.h b/include/drivers/console.h
index f499571..fa4eb94 100644
--- a/include/drivers/console.h
+++ b/include/drivers/console.h
@@ -12,10 +12,16 @@
#define CONSOLE_T_NEXT (U(0) * REGSZ)
#define CONSOLE_T_FLAGS (U(1) * REGSZ)
#define CONSOLE_T_PUTC (U(2) * REGSZ)
+#if ENABLE_CONSOLE_GETC
#define CONSOLE_T_GETC (U(3) * REGSZ)
#define CONSOLE_T_FLUSH (U(4) * REGSZ)
#define CONSOLE_T_BASE (U(5) * REGSZ)
#define CONSOLE_T_DRVDATA (U(6) * REGSZ)
+#else
+#define CONSOLE_T_FLUSH (U(3) * REGSZ)
+#define CONSOLE_T_BASE (U(4) * REGSZ)
+#define CONSOLE_T_DRVDATA (U(5) * REGSZ)
+#endif
#define CONSOLE_FLAG_BOOT (U(1) << 0)
#define CONSOLE_FLAG_RUNTIME (U(1) << 1)
@@ -42,7 +48,9 @@
*/
u_register_t flags;
int (*const putc)(int character, struct console *console);
+#if ENABLE_CONSOLE_GETC
int (*const getc)(struct console *console);
+#endif
void (*const flush)(struct console *console);
uintptr_t base;
/* Additional private driver data may follow here. */
@@ -75,8 +83,10 @@
void console_switch_state(unsigned int new_state);
/* Output a character on all consoles registered for the current state. */
int console_putc(int c);
+#if ENABLE_CONSOLE_GETC
/* Read a character (blocking) from any console registered for current state. */
int console_getc(void);
+#endif
/* Flush all consoles registered for the current state. */
void console_flush(void);
diff --git a/include/drivers/console_assertions.h b/include/drivers/console_assertions.h
index 00caa31..9f06573 100644
--- a/include/drivers/console_assertions.h
+++ b/include/drivers/console_assertions.h
@@ -19,8 +19,10 @@
assert_console_t_flags_offset_mismatch);
CASSERT(CONSOLE_T_PUTC == __builtin_offsetof(console_t, putc),
assert_console_t_putc_offset_mismatch);
+#if ENABLE_CONSOLE_GETC
CASSERT(CONSOLE_T_GETC == __builtin_offsetof(console_t, getc),
assert_console_t_getc_offset_mismatch);
+#endif
CASSERT(CONSOLE_T_FLUSH == __builtin_offsetof(console_t, flush),
assert_console_t_flush_offset_mismatch);
CASSERT(CONSOLE_T_DRVDATA == sizeof(console_t),