linux-kernel - Re: [PATCH v4 1/2] powerpc/pseries/iommu: Share the per-cpu TCE page with the hypervisor.

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Date:   Tue, 3 Dec 2019 11:56:43 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Ram Pai <linuxram@...ibm.com>, linuxppc-dev@...ts.ozlabs.org,
        mpe@...erman.id.au
Cc:     benh@...nel.crashing.org, david@...son.dropbear.id.au,
        paulus@...abs.org, mdroth@...ux.vnet.ibm.com, hch@....de,
        andmike@...ibm.com, sukadev@...ux.vnet.ibm.com, mst@...hat.com,
        ram.n.pai@...il.com, cai@....pw, tglx@...utronix.de,
        bauerman@...ux.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] powerpc/pseries/iommu: Share the per-cpu TCE page
 with the hypervisor.



On 02/12/2019 17:45, Ram Pai wrote:
> H_PUT_TCE_INDIRECT hcall uses a page filled with TCE entries, as one of
> its parameters. One page is dedicated per cpu, for the lifetime of the
> kernel for this purpose. On secure VMs, contents of this page, when
> accessed by the hypervisor, retrieves encrypted TCE entries.  Hypervisor
> needs to know the unencrypted entries, to update the TCE table
> accordingly.  There is nothing secret or sensitive about these entries.
> Hence share the page with the hypervisor.

This unsecures a page in the guest in a random place which creates an
additional attack surface which is hard to exploit indeed but
nevertheless it is there. A safer option would be not to use the
hcall-multi-tce hyperrtas option (which translates FW_FEATURE_MULTITCE
in the guest).

Also what is this for anyway? If I understand things right, you cannot
map any random guest memory, you should only be mapping that 64MB-ish
bounce buffer array but 1) I do not see that happening (I may have
missed it) 2) it should be done once and it takes a little time for
whatever memory size we allow for bounce buffers anyway. Thanks,


> 
> Signed-off-by: Ram Pai <linuxram@...ibm.com>
> ---
>  arch/powerpc/platforms/pseries/iommu.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 6ba081d..0720831 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -37,6 +37,7 @@
>  #include <asm/mmzone.h>
>  #include <asm/plpar_wrappers.h>
>  #include <asm/svm.h>
> +#include <asm/ultravisor.h>
>  
>  #include "pseries.h"
>  
> @@ -179,6 +180,23 @@ static int tce_build_pSeriesLP(struct iommu_table *tbl, long tcenum,
>  
>  static DEFINE_PER_CPU(__be64 *, tce_page);
>  
> +/*
> + * Allocate a tce page.  If secure VM, share the page with the hypervisor.
> + *
> + * NOTE: the TCE page is shared with the hypervisor explicitly and remains
> + * shared for the lifetime of the kernel. It is implicitly unshared at kernel
> + * shutdown through a UV_UNSHARE_ALL_PAGES ucall.
> + */
> +static __be64 *alloc_tce_page(void)
> +{
> +	__be64 *tcep = (__be64 *)__get_free_page(GFP_ATOMIC);
> +
> +	if (tcep && is_secure_guest())
> +		uv_share_page(PHYS_PFN(__pa(tcep)), 1);
> +
> +	return tcep;
> +}
> +
>  static int tce_buildmulti_pSeriesLP(struct iommu_table *tbl, long tcenum,
>  				     long npages, unsigned long uaddr,
>  				     enum dma_data_direction direction,
> @@ -206,8 +224,7 @@ static int tce_buildmulti_pSeriesLP(struct iommu_table *tbl, long tcenum,
>  	 * from iommu_alloc{,_sg}()
>  	 */
>  	if (!tcep) {
> -		tcep = (__be64 *)__get_free_page(GFP_ATOMIC);
> -		/* If allocation fails, fall back to the loop implementation */
> +		tcep = alloc_tce_page();
>  		if (!tcep) {
>  			local_irq_restore(flags);
>  			return tce_build_pSeriesLP(tbl, tcenum, npages, uaddr,
> @@ -405,7 +422,7 @@ static int tce_setrange_multi_pSeriesLP(unsigned long start_pfn,
>  	tcep = __this_cpu_read(tce_page);
>  
>  	if (!tcep) {
> -		tcep = (__be64 *)__get_free_page(GFP_ATOMIC);
> +		tcep = alloc_tce_page();
>  		if (!tcep) {
>  			local_irq_enable();
>  			return -ENOMEM;
> 

-- 
Alexey