RE: [PATCH v2 5/9] Drivers: hv: vmbus: Support >64 VPs for a fully enlightened TDX/SNP VM

[Michael Kelley (LINUX)]

From: Dexuan Cui <decui@xxxxxxxxxxxxx> Sent: Sunday, August 20, 2023 1:27 PM
> 
> Don't set *this_cpu_ptr(hyperv_pcpu_input_arg) before the function
> set_memory_decrypted() returns, otherwise we run into this ticky issue:
> 
> For a fully enlightened TDX/SNP VM, in hv_common_cpu_init(),
> *this_cpu_ptr(hyperv_pcpu_input_arg) is an encrypted page before
> the set_memory_decrypted() returns.
> 
> When such a VM has more than 64 VPs, if the hyperv_pcpu_input_arg is not
> NULL, hv_common_cpu_init() -> set_memory_decrypted() -> ... ->
> cpa_flush() -> on_each_cpu() -> ... -> hv_send_ipi_mask() -> ... ->
> __send_ipi_mask_ex() tries to call hv_do_fast_hypercall16() with the

Actually, it tries to call hv_do_rep_hypercall().  Using the memory-based
hyperv_pcpu_input_arg with a "fast" hypercall doesn't make sense.

> hyperv_pcpu_input_arg as the hypercall input page, which must be a
> decrypted page in such a VM, but the page is still encrypted at this
> point, and a fatal fault is triggered.
> 
> Fix the issue by setting *this_cpu_ptr(hyperv_pcpu_input_arg) after
> set_memory_decrypted(): if the hyperv_pcpu_input_arg is NULL,
> __send_ipi_mask_ex() returns HV_STATUS_INVALID_PARAMETER immediately,
> and hv_send_ipi_mask() falls back to orig_apic.send_IPI_mask(),
> which can use x2apic_send_IPI_all(), which may be slightly slower than
> the hypercall but still works correctly in such a VM.
> 
> Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> ---
> 
> Changes in v2: None
> 
>  drivers/hv/hv_common.c | 30 +++++++++++++++++++++++-------
>  1 file changed, 23 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index 897bbb96f4118..4c858e1636da7 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -360,6 +360,7 @@ int hv_common_cpu_init(unsigned int cpu)
>  	u64 msr_vp_index;
>  	gfp_t flags;
>  	int pgcount = hv_root_partition ? 2 : 1;
> +	void *mem;
>  	int ret;
> 
>  	/* hv_cpu_init() can be called with IRQs disabled from hv_resume() */
> @@ -372,25 +373,40 @@ int hv_common_cpu_init(unsigned int cpu)
>  	 * allocated if this CPU was previously online and then taken offline
>  	 */
>  	if (!*inputarg) {
> -		*inputarg = kmalloc(pgcount * HV_HYP_PAGE_SIZE, flags);
> -		if (!(*inputarg))
> +		mem = kmalloc(pgcount * HV_HYP_PAGE_SIZE, flags);
> +		if (!mem)
>  			return -ENOMEM;
> 
>  		if (hv_root_partition) {
>  			outputarg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
> -			*outputarg = (char *)(*inputarg) + HV_HYP_PAGE_SIZE;
> +			*outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
>  		}
> 
>  		if (hv_isolation_type_en_snp() || hv_isolation_type_tdx()) {
> -			ret = set_memory_decrypted((unsigned long)*inputarg, pgcount);
> +			ret = set_memory_decrypted((unsigned long)mem, pgcount);
>  			if (ret) {
> -				/* It may be unsafe to free *inputarg */
> -				*inputarg = NULL;
> +				/* It may be unsafe to free 'mem' */
>  				return ret;
>  			}
> 
> -			memset(*inputarg, 0x00, pgcount * PAGE_SIZE);
> +			memset(mem, 0x00, pgcount * HV_HYP_PAGE_SIZE);
>  		}
> +
> +		/*
> +		 * In a fully enlightened TDX/SNP VM with more than 64 VPs, if
> +		 * hyperv_pcpu_input_arg is not NULL, set_memory_decrypted() ->
> +		 * ... -> cpa_flush()-> ... -> __send_ipi_mask_ex() tries to
> +		 * use hyperv_pcpu_input_arg as the hypercall input page, which
> +		 * must be a decrypted page in such a VM, but the page is still
> +		 * encrypted before set_memory_decrypted() returns. Fix this by
> +		 * setting *inputarg after the above set_memory_decrypted(): if
> +		 * hyperv_pcpu_input_arg is NULL, __send_ipi_mask_ex() returns
> +		 * HV_STATUS_INVALID_PARAMETER immediately, and the function
> +		 * hv_send_ipi_mask() falls back to orig_apic.send_IPI_mask(),
> +		 * which may be slightly slower than the hypercall, but still
> +		 * works correctly in such a VM.
> +		 */
> +		*inputarg = mem;
>  	}
> 
>  	msr_vp_index = hv_get_register(HV_REGISTER_VP_INDEX);
> --
> 2.25.1

Modulo the minor correction in the commit message,

Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>