tree 09a6e05374bf55dbb93c070c8373f593981a7e26
parent 2744f075ea437bb27e9a1e0c4569948789f9091d
author Sandrine Bailleux <sandrine.bailleux@arm.com> 1697006280 +0200
committer Sandrine Bailleux <sandrine.bailleux@arm.com> 1697006414 +0200

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>