[PATCH v3 0/6] riscv: SCS support

[Sami Tolvanen]

Hi folks,

This series adds Shadow Call Stack (SCS) support for RISC-V. SCS
uses compiler instrumentation to store return addresses in a
separate shadow stack to protect them against accidental or
malicious overwrites. More information about SCS can be found
here:

  https://clang.llvm.org/docs/ShadowCallStack.html

Patch 1 is from Deepak, and it simplifies VMAP_STACK overflow
handling by adding support for accessing per-CPU variables
directly in assembly. The patch is included in this series to
make IRQ stack switching cleaner with SCS, and I've simply
rebased it and fixed a couple of minor issues. Patch 2 uses this
functionality to clean up the stack switching by moving duplicate
code into a single function. On RISC-V, the compiler uses the
gp register for storing the current shadow call stack pointer,
which is incompatible with global pointer relaxation. Patch 3
moves global pointer loading into a macro that can be easily
disabled with SCS. Patch 4 implements SCS register loading and
switching, and allows the feature to be enabled, and patch 5 adds
separate per-CPU IRQ shadow call stacks when CONFIG_IRQ_STACKS is
enabled. Patch 6 fixes the backward-edge CFI test in lkdtm for
RISC-V.

Note that this series requires Clang 17. Earlier Clang versions
support SCS on RISC-V, but use the x18 register instead of gp,
which isn't ideal. gcc has SCS support for arm64, but I'm not
aware of plans to support RISC-V. Once the Zicfiss extension is
ratified, it's probably preferable to use hardware-backed shadow
stacks instead of SCS on hardware that supports the extension,
and we may want to consider implementing CONFIG_DYNAMIC_SCS to
patch between the implementations at runtime (similarly to the
arm64 implementation, which switches to SCS when hardware PAC
support isn't available).

Sami

---

Changes in v3:
  - Dropped a now unneeded function declaration (patch 1).
  - Refactored call_on_irq_stack to use stack frame offsets
    based on Clément's suggestion (patch 2).
  - Rebased on top of v6.5.

Changes in v2:
  - Fixed asm_per_cpu with !CONFIG_SMP (patch 1).
  - Added a fix to the CFI_BACKWARD lkdtm test (patch 6).
  - Rebased on top of -rc6.

---

Deepak Gupta (1):
  riscv: VMAP_STACK overflow detection thread-safe

Sami Tolvanen (5):
  riscv: Deduplicate IRQ stack switching
  riscv: Move global pointer loading to a macro
  riscv: Implement Shadow Call Stack
  riscv: Use separate IRQ shadow call stacks
  lkdtm: Fix CFI_BACKWARD on RISC-V

 arch/riscv/Kconfig                      |   6 ++
 arch/riscv/Makefile                     |   4 +
 arch/riscv/include/asm/asm-prototypes.h |   1 -
 arch/riscv/include/asm/asm.h            |  41 ++++++++
 arch/riscv/include/asm/irq_stack.h      |   3 +
 arch/riscv/include/asm/scs.h            |  54 +++++++++++
 arch/riscv/include/asm/thread_info.h    |  16 ++-
 arch/riscv/kernel/asm-offsets.c         |   9 ++
 arch/riscv/kernel/entry.S               | 124 ++++++++++++------------
 arch/riscv/kernel/head.S                |  19 ++--
 arch/riscv/kernel/irq.c                 |  56 +++++------
 arch/riscv/kernel/suspend_entry.S       |   5 +-
 arch/riscv/kernel/traps.c               |  68 +------------
 arch/riscv/kernel/vdso/Makefile         |   2 +-
 arch/riscv/purgatory/Makefile           |   4 +
 drivers/misc/lkdtm/cfi.c                |  13 ++-
 16 files changed, 248 insertions(+), 177 deletions(-)
 create mode 100644 arch/riscv/include/asm/scs.h


base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
-- 
2.42.0.rc2.253.gd59a3bf2b4-goog