linux-kernel - Re: [PATCHv4] arm64: Handle el1 synchronous instruction aborts cleanly

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Message-ID: <20160809132431.GK27508@arm.com>
Date:	Tue, 9 Aug 2016 14:24:31 +0100
From:	Will Deacon <will.deacon@....com>
To:	Laura Abbott <labbott@...hat.com>
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4] arm64: Handle el1 synchronous instruction aborts
 cleanly

On Mon, Aug 08, 2016 at 05:35:34PM -0700, Laura Abbott wrote:
> Executing from a non-executable area gives an ugly message:
> 
> lkdtm: Performing direct entry EXEC_RODATA
> lkdtm: attempting ok execution at ffff0000084c0e08
> lkdtm: attempting bad execution at ffff000008880700
> Bad mode in Synchronous Abort handler detected on CPU2, code 0x8400000e -- IABT (current EL)
> CPU: 2 PID: 998 Comm: sh Not tainted 4.7.0-rc2+ #13
> Hardware name: linux,dummy-virt (DT)
> task: ffff800077e35780 ti: ffff800077970000 task.ti: ffff800077970000
> PC is at lkdtm_rodata_do_nothing+0x0/0x8
> LR is at execute_location+0x74/0x88
> 
> The 'IABT (current EL)' indicates the error but it's a bit cryptic
> without knowledge of the ARM ARM. There is also no indication of the
> specific address which triggered the fault. The increase in kernel
> page permissions makes hitting this case more likely as well.
> Handling the case in the vectors gives a much more familiar looking
> error message:
> 
> lkdtm: Performing direct entry EXEC_RODATA
> lkdtm: attempting ok execution at ffff0000084c0840
> lkdtm: attempting bad execution at ffff000008880680
> Unable to handle kernel paging request at virtual address ffff000008880680
> pgd = ffff8000089b2000
> [ffff000008880680] *pgd=00000000489b4003, *pud=0000000048904003, *pmd=0000000000000000
> Internal error: Oops: 8400000e [#1] PREEMPT SMP
> Modules linked in:
> CPU: 1 PID: 997 Comm: sh Not tainted 4.7.0-rc1+ #24
> Hardware name: linux,dummy-virt (DT)
> task: ffff800077f9f080 ti: ffff800008a1c000 task.ti: ffff800008a1c000
> PC is at lkdtm_rodata_do_nothing+0x0/0x8
> LR is at execute_location+0x74/0x88
> 
> Signed-off-by: Laura Abbott <labbott@...hat.com>
> Acked-by: Mark Rutland <mark.rutland@....com>
> ---
> v4: Rebased to master, extra error message to indicate execution of userspace
> memory
> ---
>  arch/arm64/kernel/entry.S | 18 ++++++++++++++++++
>  arch/arm64/mm/fault.c     | 14 ++++++++++++--
>  2 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 96e4a2b..bdfadef 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -353,6 +353,8 @@ el1_sync:
>  	lsr	x24, x1, #ESR_ELx_EC_SHIFT	// exception class
>  	cmp	x24, #ESR_ELx_EC_DABT_CUR	// data abort in EL1
>  	b.eq	el1_da
> +	cmp	x24, #ESR_ELx_EC_IABT_CUR	// instruction abort in EL1
> +	b.eq	el1_ia
>  	cmp	x24, #ESR_ELx_EC_SYS64		// configurable trap
>  	b.eq	el1_undef
>  	cmp	x24, #ESR_ELx_EC_SP_ALIGN	// stack alignment exception
> @@ -364,6 +366,22 @@ el1_sync:
>  	cmp	x24, #ESR_ELx_EC_BREAKPT_CUR	// debug exception in EL1
>  	b.ge	el1_dbg
>  	b	el1_inv
> +el1_ia:
> +	/*
> +	 * Instruction abort handling
> +	 */
> +	mrs	x0, far_el1
> +	enable_dbg
> +	// re-enable interrupts if they were enabled in the aborted context
> +	tbnz	x23, #7, 1f			// PSR_I_BIT
> +	enable_irq
> +1:
> +	mov	x2, sp				// struct pt_regs
> +	bl	do_mem_abort
> +
> +	// disable interrupts before pulling preserved data off the stack
> +	disable_irq
> +	kernel_exit 1

This looks identical to the el1_da code immediately below. Can we not
just have a fallthrough?

Will