Messages in this thread | | | Subject | Re: [PATCH v4 1/2] powerpc/pseries/iommu: Share the per-cpu TCE page with the hypervisor. | From | Alexey Kardashevskiy <> | Date | Tue, 3 Dec 2019 13:15:04 +1100 |
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On 03/12/2019 13:08, Ram Pai wrote: > On Tue, Dec 03, 2019 at 11:56:43AM +1100, Alexey Kardashevskiy wrote: >> >> >> 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). > > > Hmm... How do we not use it? AFAICT hcall-multi-tce option gets invoked > automatically when IOMMU option is enabled.
It is advertised by QEMU but the guest does not have to use it.
> This happens even > on a normal VM when IOMMU is enabled. > > >> >> Also what is this for anyway? > > This is for sending indirect-TCE entries to the hypervisor. > The hypervisor must be able to read those TCE entries, so that it can > use those entires to populate the TCE table with the correct mappings. > >> 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, > > Any random guest memory can be shared by the guest.
Yes but we do not want this to be this random. I thought the whole idea of swiotlb was to restrict the amount of shared memory to bare minimum, what do I miss?
> Maybe you are confusing this with the SWIOTLB bounce buffers used by PCI > devices, to transfer data to the hypervisor?
Is not this for pci+swiotlb? The cover letter suggests it is for virtio-scsi-_pci_ with iommu_platform=on which makes it a normal pci device just like emulated XHCI. 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 >
-- Alexey
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