This registers a breakpoint handler for the new breakpoint type
(0x03) inserted by LLVM CLANG for CFI breakpoints.

If we are in permissive mode, just print a backtrace and continue.

Example with CONFIG_CFI_PERMISSIVE enabled:

> echo CFI_FORWARD_PROTO > /sys/kernel/debug/provoke-crash/DIRECT
lkdtm: Performing direct entry CFI_FORWARD_PROTO
lkdtm: Calling matched prototype ...
lkdtm: Calling mismatched prototype ...
CFI failure at lkdtm_indirect_call+0x40/0x4c (target: 0x0; expected type: 0x00000000)
WARNING: CPU: 1 PID: 112 at lkdtm_indirect_call+0x40/0x4c
CPU: 1 PID: 112 Comm: sh Not tainted 6.8.0-rc1+ #150
Hardware name: ARM-Versatile Express
(...)
lkdtm: FAIL: survived mismatched prototype function call!
lkdtm: Unexpected! This kernel (6.8.0-rc1+ armv7l) was built with CONFIG_CFI_CLANG=y

As you can see the LKDTM test fails, but I expect that this would be
expected behaviour in the permissive mode.

We are currently not implementing target and type for the CFI
breakpoint as this requires additional operand bundling compiler
extensions.

CPUs without breakpoint support cannot handle breakpoints naturally,
in these cases the permissive mode will not work, CFI will fall over
on an undefined instruction:

Internal error: Oops - undefined instruction: 0 [#1] PREEMPT ARM
CPU: 0 PID: 186 Comm: ash Tainted: G        W          6.9.0-rc1+ #7
Hardware name: Gemini (Device Tree)
PC is at lkdtm_indirect_call+0x38/0x4c
LR is at lkdtm_CFI_FORWARD_PROTO+0x30/0x6c

This is reasonable I think: it's the best CFI can do to ascertain
the the control flow is not broken on these CPUs.
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>