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SubjectRe: [PATCH v4 1/2] powerpc/pseries/iommu: Share the per-cpu TCE page with the hypervisor.
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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@us.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

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