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

[Marc Zyngier <marc.zyngier@arm.com>]

On 09/07/2019 15:18, Neeraj Upadhyay wrote:
> 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?

No, I just focused on the rest of the asm code and missed it, apologies.

> 
>>
>>> +
>>>   	.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.

Yup, well spotted. I'll respin it shortly and we can then work out
whether that's really a better approach.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...