Re: [PATCH] arm64: Explicitly set pstate.ssbs for el0 on kernel entry

[Neeraj Upadhyay <neeraju@codeaurora.org>]

Hi Marc,

On 7/9/19 6:38 PM, Marc Zyngier wrote:
> Hi Neeraj,
>
> On 09/07/2019 12:22, Neeraj Upadhyay wrote:
>> For cpus which do not support pstate.ssbs feature, el0
>> might not retain spsr.ssbs. This is problematic, if this
>> task migrates to a cpu supporting this feature, thus
>> relying on its state to be correct. On kernel entry,
>> explicitly set spsr.ssbs, so that speculation is enabled
>> for el0, when this task migrates to a cpu supporting
>> ssbs feature. Restoring state at kernel entry ensures
>> that el0 ssbs state is always consistent while we are
>> in el1.
>>
>> As alternatives are applied by boot cpu, at the end of smp
>> init, presence/absence of ssbs feature on boot cpu, is used
>> for deciding, whether the capability is uniformly provided.
> I've seen the same issue, but went for a slightly different
> approach, see below.
>
>> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
>> ---
>>   arch/arm64/kernel/cpu_errata.c | 16 ++++++++++++++++
>>   arch/arm64/kernel/entry.S      | 26 +++++++++++++++++++++++++-
>>   2 files changed, 41 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
>> index ca11ff7..c84a56d 100644
>> --- a/arch/arm64/kernel/cpu_errata.c
>> +++ b/arch/arm64/kernel/cpu_errata.c
>> @@ -336,6 +336,22 @@ void __init arm64_enable_wa2_handling(struct alt_instr *alt,
>>   		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
>>   }
>>   
>> +void __init arm64_restore_ssbs_state(struct alt_instr *alt,
>> +				     __le32 *origptr, __le32 *updptr,
>> +				     int nr_inst)
>> +{
>> +	BUG_ON(nr_inst != 1);
>> +	/*
>> +	 * Only restore EL0 SSBS state on EL1 entry if cpu does not
>> +	 * support the capability and capability is present for at
>> +	 * least one cpu and if the SSBD state allows it to
>> +	 * be changed.
>> +	 */
>> +	if (!this_cpu_has_cap(ARM64_SSBS) && cpus_have_cap(ARM64_SSBS) &&
>> +	    arm64_get_ssbd_state() != ARM64_SSBD_FORCE_ENABLE)
>> +		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
>> +}
>> +
>>   void arm64_set_ssbd_mitigation(bool state)
>>   {
>>   	if (!IS_ENABLED(CONFIG_ARM64_SSBD)) {
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index 9cdc459..7e79305 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -143,6 +143,25 @@ alternative_cb_end
>>   #endif
>>   	.endm
>>   
>> +	// This macro updates spsr. It also corrupts the condition
>> +	// codes state.
>> +	.macro	restore_ssbs_state, saved_spsr, tmp
>> +#ifdef CONFIG_ARM64_SSBD
>> +alternative_cb	arm64_restore_ssbs_state
>> +	b	.L__asm_ssbs_skip\@
>> +alternative_cb_end
>> +	ldr	\tmp, [tsk, #TSK_TI_FLAGS]
>> +	tbnz	\tmp, #TIF_SSBD, .L__asm_ssbs_skip\@
>> +	tst	\saved_spsr, #PSR_MODE32_BIT	// native task?
>> +	b.ne	.L__asm_ssbs_compat\@
>> +	orr	\saved_spsr, \saved_spsr, #PSR_SSBS_BIT
>> +	b	.L__asm_ssbs_skip\@
>> +.L__asm_ssbs_compat\@:
>> +	orr	\saved_spsr, \saved_spsr, #PSR_AA32_SSBS_BIT
>> +.L__asm_ssbs_skip\@:
>> +#endif
>> +	.endm
> Although this is in keeping with the rest of entry.S (perfectly
> unreadable ;-), I think we can do something a bit simpler, that
> doesn't rely on patching. Also, this doesn't seem to take the
> SSBD options such as ARM64_SSBD_FORCE_ENABLE into account.

arm64_restore_ssbs_state has a check for ARM64_SSBD_FORCE_ENABLE,

does that look wrong?

>
>> +
>>   	.macro	kernel_entry, el, regsize = 64
>>   	.if	\regsize == 32
>>   	mov	w0, w0				// zero upper 32 bits of x0
>> @@ -182,8 +201,13 @@ alternative_cb_end
>>   	str	x20, [tsk, #TSK_TI_ADDR_LIMIT]
>>   	/* No need to reset PSTATE.UAO, hardware's already set it to 0 for us */
>>   	.endif /* \el == 0 */
>> -	mrs	x22, elr_el1
>>   	mrs	x23, spsr_el1
>> +
>> +	.if	\el == 0
>> +	restore_ssbs_state x23, x22
>> +	.endif
>> +
>> +	mrs	x22, elr_el1
>>   	stp	lr, x21, [sp, #S_LR]
>>   
>>   	/*
>>
> How about the patch below?

Looks good; was just going to mention PF_KTHREAD check, but Mark R. has 
already

given detailed information about it.


Thanks

Neeraj

>
> Thanks,
>
> 	M.
>
>  From 7d4314d1ef3122d8bf56a7ef239c8c68e0c81277 Mon Sep 17 00:00:00 2001
> From: Marc Zyngier <marc.zyngier@arm.com>
> Date: Tue, 4 Jun 2019 17:35:18 +0100
> Subject: [PATCH] arm64: Force SSBS on context switch
>
> On a CPU that doesn't support SSBS, PSTATE[12] is RES0.  In a system
> where only some of the CPUs implement SSBS, we end-up losing track of
> the SSBS bit across task migration.
>
> To address this issue, let's force the SSBS bit on context switch.
>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
>   arch/arm64/include/asm/processor.h | 14 ++++++++++++--
>   arch/arm64/kernel/process.c        | 14 ++++++++++++++
>   2 files changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index fd5b1a4efc70..844e2964b0f5 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -193,6 +193,16 @@ static inline void start_thread_common(struct pt_regs *regs, unsigned long pc)
>   		regs->pmr_save = GIC_PRIO_IRQON;
>   }
>   
> +static inline void set_ssbs_bit(struct pt_regs *regs)
> +{
> +	regs->pstate |= PSR_SSBS_BIT;
> +}
> +
> +static inline void set_compat_ssbs_bit(struct pt_regs *regs)
> +{
> +	regs->pstate |= PSR_AA32_SSBS_BIT;
> +}
> +
>   static inline void start_thread(struct pt_regs *regs, unsigned long pc,
>   				unsigned long sp)
>   {
> @@ -200,7 +210,7 @@ static inline void start_thread(struct pt_regs *regs, unsigned long pc,
>   	regs->pstate = PSR_MODE_EL0t;
>   
>   	if (arm64_get_ssbd_state() != ARM64_SSBD_FORCE_ENABLE)
> -		regs->pstate |= PSR_SSBS_BIT;
> +		set_ssbs_bit(regs);
>   
>   	regs->sp = sp;
>   }
> @@ -219,7 +229,7 @@ static inline void compat_start_thread(struct pt_regs *regs, unsigned long pc,
>   #endif
>   
>   	if (arm64_get_ssbd_state() != ARM64_SSBD_FORCE_ENABLE)
> -		regs->pstate |= PSR_AA32_SSBS_BIT;
> +		set_compat_ssbs_bit(regs);
>   
>   	regs->compat_sp = sp;
>   }
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 9856395ccdb7..d451b3b248cf 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -442,6 +442,19 @@ void uao_thread_switch(struct task_struct *next)
>   	}
>   }
>   
> +static void ssbs_thread_switch(struct task_struct *next)
> +{
> +	if (arm64_get_ssbd_state() != ARM64_SSBD_FORCE_ENABLE &&
> +	    !test_tsk_thread_flag(next, TIF_SSBD)) {
> +		struct pt_regs *regs = task_pt_regs(next);
> +
> +		if (compat_user_mode(regs))
> +			set_compat_ssbs_bit(regs);
> +		else if (user_mode(regs))
> +			set_ssbs_bit(regs);
> +	}
> +}
> +
>   /*
>    * We store our current task in sp_el0, which is clobbered by userspace. Keep a
>    * shadow copy so that we can restore this upon entry from userspace.
> @@ -471,6 +484,7 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
>   	entry_task_switch(next);
>   	uao_thread_switch(next);
>   	ptrauth_thread_switch(next);
> +	ssbs_thread_switch(next);
>   
>   	/*
>   	 * Complete any pending TLB or cache maintenance on this CPU in case

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