From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 7625C1DDC3F; Fri, 3 Jul 2026 09:27:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783070858; cv=none; b=mzvF4ON/rD0AwECFeZYasl6VlzxTDHfyuUH1nBSSEtaFISdRN1rK8CnjVU7wYDrHvO433U79ST2cVRQdQz7FXgtBstpRRxfqXPpeaO2nc3gwkswy8xHmSjL1HNnVmcfAbDj5U4lRJCCDlXyf2Nu9uIF2R5ZzExPZhCtBRAErl1M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783070858; c=relaxed/simple; bh=JvoU5nbICVMIWRHb0kIPGUpPkes8KJNpKqGQHTJwF10=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=mnDdu1FZIHoFlFzEEJPNU4dHMXjRegCjW+M7vpykqJnqpCxFjT+929duPZ0mP4jL+TMb4Qpbu2LFWVJNxvKJZqqUbw+AjvXG9HxBK0ssW46EHd4qkF0dLLuglrfnSfZt3kpvwlpZ7PG8VzNx3Pkqu2wWQnRe0KAYVg1o8WOOh7E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=ikRQDQNa; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="ikRQDQNa" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id BFE8F1EDB; Fri, 3 Jul 2026 02:27:27 -0700 (PDT) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7F0003F673; Fri, 3 Jul 2026 02:27:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1783070852; bh=JvoU5nbICVMIWRHb0kIPGUpPkes8KJNpKqGQHTJwF10=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ikRQDQNahGHiJFINP6/dnNmuIEPdANlrfpfBFrGxrRs/b4KK68wYKWhOXNc+EJNEa vFWsp4D+IrAq/iI69qM2zxoeC9EXqgUPCoYNtcZ2cIZu8S4wc6UUpcRfTYT+KhIOPl THZ9ggf4tGc+y83nvTHAdrGYDpeQhXtGYAoR248Q= Date: Fri, 3 Jul 2026 10:27:28 +0100 From: Mark Rutland To: Linus Walleij Cc: Russell King , Nathan Chancellor , Sami Tolvanen , Kees Cook , "Russell King (Oracle)" , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, slipher Subject: Re: [PATCH v3] ARM: breakpoint: CFI breakpoints only on demand Message-ID: References: <20260701-arm32-cfi-bug-v3-1-e3c37e2b80a4@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260701-arm32-cfi-bug-v3-1-e3c37e2b80a4@kernel.org> 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 > Closes: https://lore.kernel.org/lkml/kJqktbpLphg_Pk5I5SPptgTLjl3E3eq5mN5UzCslyFj7Q1Irp-wDid4mj5eQVd2iZtRGXgeZd8goq195EkXdjyt864YMc8mVb2B9NGH91NQ=@protonmail.com/ > Signed-off-by: Linus Walleij > --- > 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 > >