Merge patch series "riscv: KCFI support"

Sami Tolvanen <samitolvanen@google.com> says:

This series adds KCFI support for RISC-V. KCFI is a fine-grained
forward-edge control-flow integrity scheme supported in Clang >=16,
which ensures indirect calls in instrumented code can only branch to
functions whose type matches the function pointer type, thus making
code reuse attacks more difficult.

Patch 1 implements a pt_regs based syscall wrapper to address
function pointer type mismatches in syscall handling. Patches 2 and 3
annotate indirectly called assembly functions with CFI types. Patch 4
implements error handling for indirect call checks. Patch 5 disables
CFI for arch/riscv/purgatory. Patch 6 finally allows CONFIG_CFI_CLANG
to be enabled for RISC-V.

Note that Clang 16 has a generic architecture-agnostic KCFI
implementation, which does work with the kernel, but doesn't produce
a stable code sequence for indirect call checks, which means
potential failures just trap and won't result in informative error
messages. Clang 17 includes a RISC-V specific back-end implementation
for KCFI, which emits a predictable code sequence for the checks and a
.kcfi_traps section with locations of the traps, which patch 5 uses to
produce more useful errors.

The type mismatch fixes and annotations in the first three patches
also become necessary in future if the kernel decides to support
fine-grained CFI implemented using the hardware landing pad
feature proposed in the in-progress Zicfisslp extension. Once the
specification is ratified and hardware support emerges, implementing
runtime patching support that replaces KCFI instrumentation with
Zicfisslp landing pads might also be feasible (similarly to KCFI to
FineIBT patching on x86_64), allowing distributions to ship a unified
kernel binary for all devices.

* b4-shazam-merge:
  riscv: Allow CONFIG_CFI_CLANG to be selected
  riscv/purgatory: Disable CFI
  riscv: Add CFI error handling
  riscv: Add ftrace_stub_graph
  riscv: Add types to indirectly called assembly functions
  riscv: Implement syscall wrappers

Link: https://lore.kernel.org/r/20230710183544.999540-8-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index afa7160b136c..1494bca1b53a 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -35,6 +35,7 @@ config RISCV
 	select ARCH_HAS_SET_MEMORY if MMU
 	select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
 	select ARCH_HAS_STRICT_MODULE_RWX if MMU && !XIP_KERNEL
+	select ARCH_HAS_SYSCALL_WRAPPER
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
 	select ARCH_HAS_VDSO_DATA
@@ -42,12 +43,14 @@ config RISCV
 	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
 	select ARCH_STACKWALK
 	select ARCH_SUPPORTS_ATOMIC_RMW
+	select ARCH_SUPPORTS_CFI_CLANG
 	select ARCH_SUPPORTS_DEBUG_PAGEALLOC if MMU
 	select ARCH_SUPPORTS_HUGETLBFS if MMU
 	select ARCH_SUPPORTS_PAGE_TABLE_CHECK if MMU
 	select ARCH_SUPPORTS_PER_VMA_LOCK if MMU
 	select ARCH_USE_MEMTEST
 	select ARCH_USE_QUEUED_RWLOCKS
+	select ARCH_USES_CFI_TRAPS if CFI_CLANG
 	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
 	select ARCH_WANT_FRAME_POINTERS
 	select ARCH_WANT_GENERAL_HUGETLB if !RISCV_ISA_SVNAPOT