Re: [PATCH v3] ARM: breakpoint: CFI breakpoints only on demand

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Jul 01, 2026 at 12:42:09PM +0200, Linus Walleij wrote:
> 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, we fall through to return 1 from
> hw_breakpoint_pending() "unhandled fault" so userspace can make use
> of this breakpoint.
> 
> This of course does not work if userspace want to use CFI and custom
> breakpoints at the same time, and CONFIG_CFI does exist as something
> users might want to select for their kernel. If this is not good
> acceptable we need to think about other ways for CFI to interfer, such
> as not using BKPT at all (rather something like BUG()) and back out
> the offending patch until the compiler behaviour has changed.
> 
> Fixes: c3f89986fde7 ("ARM: 9391/2: hw_breakpoint: Handle CFI breakpoints")
> Reported-by: slipher <slipher@protonmail.com>
> Closes: https://lore.kernel.org/lkml/kJqktbpLphg_Pk5I5SPptgTLjl3E3eq5mN5UzCslyFj7Q1Irp-wDid4mj5eQVd2iZtRGXgeZd8goq195EkXdjyt864YMc8mVb2B9NGH91NQ=@protonmail.com/
> 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 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
> ---
>  arch/arm/kernel/hw_breakpoint.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
> index cd4b34c96e35..007023db6a5d 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 +960,11 @@ 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);
>  		break;
> +#endif

As commented on v2, I don't think this is the right fix.

I think you should look at which privilege level the exception was taken
from (e.g. useing user_mode(regs), such that a BKPT from user mode never
results in a call into hw_breakpoint_cfi_handler(), an can be treated as
unhandled.

That way the user mode behaviour would be consistent regardless of
CONFIG_CFI, and even when CONFIG_CFI=y, user mode cannot cause the
kernel to die() by executing a BKPT.

Mark.

>  	default:
>  		ret = 1; /* Unhandled fault. */
>  	}
> 
> ---
> base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6
> change-id: 20260626-arm32-cfi-bug-10fb960749c4
> 
> Best regards,
> --  
> Linus Walleij <linusw@kernel.org>
> 
>