[RFC PATCH 0/2] Add support for Supervisor Software Events extension

[Deepak Gupta]

On Thu, Nov 30, 2023 at 10:13:17AM +0100, Clément Léger wrote:
>The SBI Supervisor Software Events (SSE) extensions provides a mechanism
>to inject software events from an SBI implementation to supervisor
>software such that it preempts all other supervisor level traps and
>interrupts [1].
>
>Various events are defined and can be send asynchronously to supervisor
>software (RAS, PMU, DEBUG, Asynchronous page fault) from SBI as well
>as platform specific events. Events can be either local (per-hart) or
>global. Events can be nested on top of each other based on priority and
>can interrupt the supervisor mode at any time.
>
>This PR adds support for this extension. First commit modifies interrupts
>handling to pass the sbi traps regs through interrupt handlers in order to inject
>SSE events from this context. Second one add the SSE support itself.

Cross-posting my response from github


I've a general comment on design of SSE which interferes with security.
This is kind of a TOCTOU issue between kernel (less priv) and opensbi (more priv)

If a control flow integrity scheme (software or hardware assisted) is implemented
to ensure control safety of kernel indirect branches, then SSE provides a mechanism
using an attacker can exercise free branches. I believe software base kCFI support
for riscv is already in mainline.

During time of event registration, kernel provides physical address of context. This
context structure acts as context to be used for event inject in S (and along with PC,
SP and other registers) Context to be used where interrupted context of S will be saved
away so that it can be restored later once injected event has finished its course This
entire context buffer is mapped in kernel space and thus is vulnerable to memory corruption.
If any CFI scheme is implemented, its expected that an attacker has "write what anywhere" bug.
Thus this design allows an attacker to corrupt this context and use mechanisms to trigger events
that will make M mode to inject events in kernel and thereby allowing free branches (which goes
against CFI schemes ) in kernel.

Without understanding limitations of opensbi, I would like to propose a solution

During registration, opensbi makes a copy of context structure.
Similarly interrupted context can also live in protected region in M mode memory
Let me know what you think.

>
>A RFC for Linux which uses this extension is available at [2].
>
>Link: https://lists.riscv.org/g/tech-prs/message/515 [1]
>Link: https://lore.kernel.org/linux-riscv/20231026143122.279437-1-cleger@rivosinc.com/ [2]
>
>Clément Léger (2):
>  lib: sbi: provides regs to sbi_ipi_process()
>  lib: sbi: add support for Supervisor Software Events extension
>
> include/sbi/sbi_ecall_interface.h |  36 +-
> include/sbi/sbi_error.h           |   4 +
> include/sbi/sbi_ipi.h             |   6 +-
> include/sbi/sbi_sse.h             | 222 +++++++
> lib/sbi/Kconfig                   |   4 +
> lib/sbi/objects.mk                |   4 +
> lib/sbi/sbi_ecall.c               |   7 +-
> lib/sbi/sbi_ecall_sse.c           |  61 ++
> lib/sbi/sbi_init.c                |  13 +
> lib/sbi/sbi_ipi.c                 |  12 +-
> lib/sbi/sbi_sse.c                 | 963 ++++++++++++++++++++++++++++++
> lib/sbi/sbi_tlb.c                 |   2 +-
> lib/sbi/sbi_trap.c                |   4 +-
> lib/utils/irqchip/imsic.c         |   2 +-
> 14 files changed, 1327 insertions(+), 13 deletions(-)
> create mode 100644 include/sbi/sbi_sse.h
> create mode 100644 lib/sbi/sbi_ecall_sse.c
> create mode 100644 lib/sbi/sbi_sse.c
>
>-- 
>2.42.0
>