[PATCH v2 9/9] ARM: KCFI: Allow permissive CFI mode
Linus Walleij
linus.walleij at linaro.org
Thu Mar 7 06:22:08 PST 2024
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:
root at Vexpress:/ 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 ...
hw-breakpoint: Permissive CFI breakpoint
CPU: 0 PID: 114 Comm: sh Not tainted 6.8.0-rc1+ #111
Hardware name: ARM-Versatile Express
unwind_backtrace from show_stack+0x28/0x30
(...)
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.
Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
---
arch/arm/include/asm/hw_breakpoint.h | 1 +
arch/arm/kernel/hw_breakpoint.c | 10 ++++++++++
2 files changed, 11 insertions(+)
diff --git a/arch/arm/include/asm/hw_breakpoint.h b/arch/arm/include/asm/hw_breakpoint.h
index 62358d3ca0a8..e7f9961c53b2 100644
--- a/arch/arm/include/asm/hw_breakpoint.h
+++ b/arch/arm/include/asm/hw_breakpoint.h
@@ -84,6 +84,7 @@ static inline void decode_ctrl_reg(u32 reg,
#define ARM_DSCR_MOE(x) ((x >> 2) & 0xf)
#define ARM_ENTRY_BREAKPOINT 0x1
#define ARM_ENTRY_ASYNC_WATCHPOINT 0x2
+#define ARM_ENTRY_CFI_BREAKPOINT 0x3
#define ARM_ENTRY_SYNC_WATCHPOINT 0xa
/* DSCR monitor/halting bits. */
diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index dc0fb7a81371..256146684813 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c
@@ -932,6 +932,16 @@ static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr,
case ARM_ENTRY_SYNC_WATCHPOINT:
watchpoint_handler(addr, fsr, regs);
break;
+ case ARM_ENTRY_CFI_BREAKPOINT:
+ if (IS_ENABLED(CONFIG_CFI_PERMISSIVE)) {
+ pr_err("Permissive CFI breakpoint\n");
+ dump_stack();
+ /* Skip the breaking instruction */
+ instruction_pointer(regs) += 4;
+ } else {
+ die("Oops - CFI", regs, 0);
+ }
+ break;
default:
ret = 1; /* Unhandled fault. */
}
--
2.34.1
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