Re: [PATCH] arm64: entry: SP Alignment Fault doesn't write to FAR_EL1

[Will Deacon <will@kernel.org>]

Hi James,

On Wed, Jul 17, 2019 at 05:56:02PM +0100, James Morse wrote:
> Comparing the arm-arm's  pseudocode for AArch64.PCAlignmentFault() with
> AArch64.SPAlignmentFault() shows that SP faults don't copy the faulty-SP
> to FAR_EL1, but this is where we read from, and the address we provide
> to user-space with the BUS_ADRALN signal.
> 
> This value will be UNKNOWN due to the previous ERET to user-space.
> If the last value is preserved, on systems with KASLR or KPTI this will
> be the user-space link-register left in FAR_EL1 by tramp_exit().
> 
> Fix this to retrieve the original sp_el0 value, and pass this to
> do_sp_pc_fault().
> 
> Fixes: 60ffc30d5652 ("arm64: Exception handling")
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
>  arch/arm64/kernel/entry.S | 25 ++++++++++++++++++++-----
>  1 file changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 165da78815c5..023e533c537e 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -743,9 +743,9 @@ el0_sync:
>  	ccmp	x24, #ESR_ELx_EC_WFx, #4, ne
>  	b.eq	el0_sys
>  	cmp	x24, #ESR_ELx_EC_SP_ALIGN	// stack alignment exception
> -	b.eq	el0_sp_pc
> +	b.eq	el0_sp
>  	cmp	x24, #ESR_ELx_EC_PC_ALIGN	// pc alignment exception
> -	b.eq	el0_sp_pc
> +	b.eq	el0_pc
>  	cmp	x24, #ESR_ELx_EC_UNKNOWN	// unknown exception in EL0
>  	b.eq	el0_undef
>  	cmp	x24, #ESR_ELx_EC_BREAKPT_LOW	// debug exception in EL0
> @@ -769,7 +769,7 @@ el0_sync_compat:
>  	cmp	x24, #ESR_ELx_EC_FP_EXC32	// FP/ASIMD exception
>  	b.eq	el0_fpsimd_exc
>  	cmp	x24, #ESR_ELx_EC_PC_ALIGN	// pc alignment exception
> -	b.eq	el0_sp_pc
> +	b.eq	el0_pc
>  	cmp	x24, #ESR_ELx_EC_UNKNOWN	// unknown exception in EL0
>  	b.eq	el0_undef
>  	cmp	x24, #ESR_ELx_EC_CP15_32	// CP15 MRC/MCR trap
> @@ -869,9 +869,24 @@ el0_fpsimd_exc:
>  	mov	x1, sp
>  	bl	do_fpsimd_exc
>  	b	ret_to_user
> -el0_sp_pc:
> +el0_sp:
>  	/*
> -	 * Stack or PC alignment exception handling
> +	 * Stack alignment exception handling
> +	 */
> +	gic_prio_kentry_setup tmp=x0
> +	enable_da_f
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +	bl	trace_hardirqs_off
> +#endif
> +	ct_user_exit
> +	ldr	x0, [sp, #S_SP]
> +	mov	x1, x25
> +	mov	x2, sp
> +	bl	do_sp_pc_abort
> +	b	ret_to_user
> +el0_pc:
> +	/*
> +	 * PC alignment exception handling

Given that this really isn't a fast path, I think it's preferable to avoid
duplicating the entry code and instead just have something like:

@@ -858,11 +858,15 @@ el0_fpsimd_exc:
 	mov	x1, sp
 	bl	do_fpsimd_exc
 	b	ret_to_user
+el0_sp:
+	ldr	x26, [sp, #S_SP]
+	b	el0_sp_pc
+el0_pc:
+	mrs	x26, far_el1
 el0_sp_pc:
 	/*
 	 * Stack or PC alignment exception handling
 	 */
-	mrs	x26, far_el1
 	gic_prio_kentry_setup tmp=x0
 	enable_da_f
 #ifdef CONFIG_TRACE_IRQFLAGS

I also think we should do the same thing for the EL1 case, even though
the address is currently ignored by the C handler.

What do you reckon?

Will

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