From 8ed9bff906cf8036531d1559f10e82733a52b41f Mon Sep 17 00:00:00 2001 From: Linus Walleij Date: Sat, 4 Jul 2026 21:15:55 +0100 Subject: [PATCH] ARM: 9481/2: 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. Closes: https://lore.kernel.org/lkml/kJqktbpLphg_Pk5I5SPptgTLjl3E3eq5mN5UzCslyFj7Q1Irp-wDid4mj5eQVd2iZtRGXgeZd8goq195EkXdjyt864YMc8mVb2B9NGH91NQ=@protonmail.com/ Fixes: c3f89986fde7 ("ARM: 9391/2: hw_breakpoint: Handle CFI breakpoints") Reported-by: slipher Suggested-by: Mark Rutland Signed-off-by: Linus Walleij Signed-off-by: Russell King --- 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. */ }