[PATCH v4] ARM: breakpoint: CFI breakpoints only on demand

This removes the stub hw_breakpoint_cfi_handler() from ARM, making
it not steal breakpoint type 0x03 (ARM_ENTRY_CFI_BREAKPOINT) unless
CFI is actively used in the kernel.

When not instrumenting with CFI, or when a breakpoint is issued in
userspace, we fall through to return 1 from hw_breakpoint_pending()
"unhandled fault" so userspace can make use of this breakpoint.

Tested with LKDTM and this command line:
echo CFI_FORWARD_PROTO > /sys/kernel/debug/provoke-crash/DIRECT
still works as expected.

Fixes: c3f89986fde7 ("ARM: 9391/2: hw_breakpoint: Handle CFI breakpoints")
Reported-by: slipher <redacted>
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Closes: https://lore.kernel.org/lkml/kJqktbpLphg_Pk5I5SPptgTLjl3E3eq5mN5UzCslyFj7Q1Irp-wDid4mj5eQVd2iZtRGXgeZd8goq195EkXdjyt864YMc8mVb2B9NGH91NQ=@protonmail.com/ (local)
Signed-off-by: Linus Walleij <linusw@kernel.org>
---
Trying to solve the CFI bug. Let's see of this first
approach is acceptable for the reporter.
---
Changes in v4:
- Dodge the BKPT if we are coming from userspace!
- Would be great if the reporter can test this with and without
  CONFIG_CFI.
- Link to v3: https://patch.msgid.link/20260701-arm32-cfi-bug-v3-1-e3c37e2b80a4@kernel.org

Changes in v3:
- Actually strip the RFC prefix...
- Link to v2: https://patch.msgid.link/20260701-arm32-cfi-bug-v2-1-9bf922593e00@kernel.org

Changes in v2:
- Resending as non-RFC so it can be applied as a band-aid.
- Link to v1: https://patch.msgid.link/20260626-arm32-cfi-bug-v1-1-a467b5050c0b@kernel.org

To: Will Deacon <will@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
To: Russell King <linux@armlinux.org.uk>
To: Kees Cook <kees@kernel.org>
To: Sami Tolvanen <samitolvanen@google.com>
To: "Russell King (Oracle)" <redacted>
To: Linus Walleij <linusw@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-perf-users@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 arch/arm/kernel/hw_breakpoint.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index cd4b34c96e35..38feb30dfb5f 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c

@@ -929,10 +929,6 @@ static void hw_breakpoint_cfi_handler(struct pt_regs *regs)
 		break;
 	}
 }
-#else
-static void hw_breakpoint_cfi_handler(struct pt_regs *regs)
-{
-}
 #endif
 
 /*

@@ -964,9 +960,14 @@ static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr,
 	case ARM_ENTRY_SYNC_WATCHPOINT:
 		watchpoint_handler(addr, fsr, regs);
 		break;
+#ifdef CONFIG_CFI
 	case ARM_ENTRY_CFI_BREAKPOINT:
-		hw_breakpoint_cfi_handler(regs);
+		if (user_mode(regs))
+			ret = 1; /* Don't handle userspace BKPT */
+		else
+			hw_breakpoint_cfi_handler(regs);
 		break;
+#endif
 	default:
 		ret = 1; /* Unhandled fault. */
 	}

---
base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6
change-id: 20260626-arm32-cfi-bug-10fb960749c4

Best regards,
--  
Linus Walleij [off-list ref]