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Message-ID: <2edc7c71-935e-0185-e547-587170a054ff@amd.com>
Date: Mon, 2 Oct 2023 13:59:03 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Michael Kelley <mikelley@...rosoft.com>, kys@...rosoft.com,
haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, hpa@...or.com, luto@...nel.org,
peterz@...radead.org, sathyanarayanan.kuppuswamy@...ux.intel.com,
kirill.shutemov@...ux.intel.com, seanjc@...gle.com,
rick.p.edgecombe@...el.com, linux-kernel@...r.kernel.org,
linux-hyperv@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 3/5] x86/mm: Mark CoCo VM pages not present while changing
encrypted state
On 10/2/23 11:35, Tom Lendacky wrote:
> On 9/29/23 13:19, Michael Kelley wrote:
>> In a CoCo VM when a page transitions from encrypted to decrypted, or vice
>> versa, attributes in the PTE must be updated *and* the hypervisor must
>> be notified of the change. Because there are two separate steps, there's
>> a window where the settings are inconsistent. Normally the code that
>> initiates the transition (via set_memory_decrypted() or
>> set_memory_encrypted()) ensures that the memory is not being accessed
>> during a transition, so the window of inconsistency is not a problem.
>> However, the load_unaligned_zeropad() function can read arbitrary memory
>> pages at arbitrary times, which could access a transitioning page during
>> the window. In such a case, CoCo VM specific exceptions are taken
>> (depending on the CoCo architecture in use). Current code in those
>> exception handlers recovers and does "fixup" on the result returned by
>> load_unaligned_zeropad(). Unfortunately, this exception handling can't
>> work in paravisor scenarios (TDX Paritioning and SEV-SNP in vTOM mode).
>> The exceptions need to be forwarded from the paravisor to the Linux
>> guest, but there are no architectural specs for how to do that.
>>
>> Fortunately, there's a simpler way to solve the problem by changing
>> the core transition code in __set_memory_enc_pgtable() to do the
>> following:
>>
>> 1. Remove aliasing mappings
>> 2. Flush the data cache if needed
>> 3. Remove the PRESENT bit from the PTEs of all transitioning pages
>> 4. Set/clear the encryption attribute as appropriate
>> 5. Flush the TLB so the changed encryption attribute isn't visible
>> 6. Notify the hypervisor of the encryption status change
>
> Not sure why I didn't notice this before, but I will need to test this to
> be certain. As part of this notification, the SNP support will issue a
> PVALIDATE instruction (to either validate or rescind validation to the
> page). PVALIDATE takes a virtual address. If the PRESENT bit has been
> removed, the PVALIDATE instruction will take a #PF (see comments below).
Yes, this series results in a #PF booting an SNP guest:
[ 0.807735] BUG: unable to handle page fault for address: ffff88803bef7000
[ 0.807829] #PF: supervisor read access in kernel mode
[ 0.807829] #PF: error_code(0x0000) - not-present page
[ 0.807829] PGD 8000004c01067 P4D 8000004c01067 PUD 8000004c02067 PMD 80001001f8063 PTE 8007ffffc4108062
[ 0.807829] Oops: 0000 [#1] PREEMPT SMP NOPTI
[ 0.807829] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.6.0-rc3-sos-testing #1
[ 0.807829] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 2/2/2022
[ 0.807829] RIP: 0010:pvalidate_pages+0x99/0x140
[ 0.807829] Code: 48 09 ca 0f b6 4c c7 0f 48 09 d6 0f b6 54 c7 0e 48 c1 e6 0c 4c 21 de 83 e2 f0 80 fa 10 4a 8d 2c 16 0f 94 c2 48 89 e8 83 e1 01 <f2> 0f 01 ff 41 89 c4 72 6
1 83 f8 06 75 45 84 c9 74 41 48 01 de 48
[ 0.807829] RSP: 0000:ffffffff82803bd0 EFLAGS: 00010246
[ 0.807829] RAX: ffff88803bef7000 RBX: ffff888000200000 RCX: 0000000000000000
[ 0.807829] RDX: 0000000000000000 RSI: 000000003bef7000 RDI: ffffffff82803c40
[ 0.807829] RBP: ffff88803bef7000 R08: 0000000000000000 R09: 0000000000000000
[ 0.807829] R10: ffff888000000000 R11: 000000ffffffffff R12: 0000000000000040
[ 0.807829] R13: 0000000000000020 R14: ffffffff82803e48 R15: ffff88803bf37000
[ 0.807829] FS: 0000000000000000(0000) GS:ffff88846fc00000(0000) knlGS:0000000000000000
[ 0.807829] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 0.807829] CR2: ffff88803bef7000 CR3: 000800000382e000 CR4: 00000000003506f0
[ 0.807829] Call Trace:
[ 0.807829] <TASK>
[ 0.807829] ? __die+0x1f/0x70
[ 0.807829] ? page_fault_oops+0x81/0x150
[ 0.807829] ? srso_return_thunk+0x5/0x5f
[ 0.807829] ? kernelmode_fixup_or_oops+0x84/0x110
[ 0.807829] ? exc_page_fault+0xa8/0x150
[ 0.807829] ? asm_exc_page_fault+0x22/0x30
[ 0.807829] ? pvalidate_pages+0x99/0x140
[ 0.807829] __set_pages_state+0x280/0x2b0
[ 0.807829] set_pages_state+0x4e/0xa0
[ 0.807829] amd_enc_status_change_finish+0x4a/0x80
[ 0.807829] __set_memory_enc_dec+0xe1/0x190
[ 0.807829] mem_encrypt_init+0x15/0xc0
[ 0.807829] start_kernel+0x31b/0x5e0
[ 0.807829] x86_64_start_reservations+0x14/0x30
[ 0.807829] x86_64_start_kernel+0x79/0x80
[ 0.807829] secondary_startup_64_no_verify+0x16b/0x16b
[ 0.807829] </TASK>
[ 0.807829] Modules linked in:
[ 0.807829] CR2: ffff88803bef7000
[ 0.807829] ---[ end trace 0000000000000000 ]---
[ 0.807829] RIP: 0010:pvalidate_pages+0x99/0x140
[ 0.807829] Code: 48 09 ca 0f b6 4c c7 0f 48 09 d6 0f b6 54 c7 0e 48 c1 e6 0c 4c 21 de 83 e2 f0 80 fa 10 4a 8d 2c 16 0f 94 c2 48 89 e8 83 e1 01 <f2> 0f 01 ff 41 89 c4 72 6
1 83 f8 06 75 45 84 c9 74 41 48 01 de 48
[ 0.807829] RSP: 0000:ffffffff82803bd0 EFLAGS: 00010246
[ 0.807829] RAX: ffff88803bef7000 RBX: ffff888000200000 RCX: 0000000000000000
[ 0.807829] RDX: 0000000000000000 RSI: 000000003bef7000 RDI: ffffffff82803c40
[ 0.807829] RBP: ffff88803bef7000 R08: 0000000000000000 R09: 0000000000000000
[ 0.807829] R10: ffff888000000000 R11: 000000ffffffffff R12: 0000000000000040
[ 0.807829] R13: 0000000000000020 R14: ffffffff82803e48 R15: ffff88803bf37000
[ 0.807829] FS: 0000000000000000(0000) GS:ffff88846fc00000(0000) knlGS:0000000000000000
[ 0.807829] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 0.807829] CR2: ffff88803bef7000 CR3: 000800000382e000 CR4: 00000000003506f0
[ 0.807829] Kernel panic - not syncing: Fatal exception
[ 0.807829] ---[ end Kernel panic - not syncing: Fatal exception ]---
Thanks,
Tom
>
>> 7. Add back the PRESENT bit, making the changed attribute visible
>>
>> With this approach, load_unaligned_zeropad() just takes its normal
>> page-fault-based fixup path if it touches a page that is transitioning.
>> As a result, load_unaligned_zeropad() and CoCo VM page transitioning
>> are completely decoupled. CoCo VM page transitions can proceed
>> without needing to handle architecture-specific exceptions and fix
>> things up. This decoupling reduces the complexity due to separate
>> TDX and SEV-SNP fixup paths, and gives more freedom to revise and
>> introduce new capabilities in future versions of the TDX and SEV-SNP
>> architectures. Paravisor scenarios work properly without needing
>> to forward exceptions.
>>
>> With this approach, the order of updating the guest PTEs and
>> notifying the hypervisor doesn't matter. As such, only a single
>> hypervisor callback is needed, rather one before and one after
>> the PTE update. Simplify the code by eliminating the extra
>> hypervisor callback along with the TDX and SEV-SNP code that
>> handles the before and after cases. The TLB flush callback is
>> also no longer required and is removed.
>>
>> Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
>> ---
>> arch/x86/coco/tdx/tdx.c | 66
>> +------------------------------------------
>> arch/x86/hyperv/ivm.c | 6 ----
>> arch/x86/kernel/x86_init.c | 4 ---
>> arch/x86/mm/mem_encrypt_amd.c | 27 ++++--------------
>> arch/x86/mm/pat/set_memory.c | 55 +++++++++++++++++++++++-------------
>> 5 files changed, 43 insertions(+), 115 deletions(-)
>>
>> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
>> index 3e6dbd2..1bb2fff 100644
>> --- a/arch/x86/coco/tdx/tdx.c
>> +++ b/arch/x86/coco/tdx/tdx.c
>> @@ -676,24 +676,6 @@ bool tdx_handle_virt_exception(struct pt_regs
>> *regs, struct ve_info *ve)
>> return true;
>> }
>> -static bool tdx_tlb_flush_required(bool private)
>> -{
>> - /*
>> - * TDX guest is responsible for flushing TLB on private->shared
>> - * transition. VMM is responsible for flushing on shared->private.
>> - *
>> - * The VMM _can't_ flush private addresses as it can't generate PAs
>> - * with the guest's HKID. Shared memory isn't subject to integrity
>> - * checking, i.e. the VMM doesn't need to flush for its own
>> protection.
>> - *
>> - * There's no need to flush when converting from shared to private,
>> - * as flushing is the VMM's responsibility in this case, e.g. it must
>> - * flush to avoid integrity failures in the face of a buggy or
>> - * malicious guest.
>> - */
>> - return !private;
>> -}
>> -
>> static bool tdx_cache_flush_required(void)
>> {
>> /*
>> @@ -776,30 +758,6 @@ static bool tdx_enc_status_changed(unsigned long
>> vaddr, int numpages, bool enc)
>> return true;
>> }
>> -static bool tdx_enc_status_change_prepare(unsigned long vaddr, int
>> numpages,
>> - bool enc)
>> -{
>> - /*
>> - * Only handle shared->private conversion here.
>> - * See the comment in tdx_early_init().
>> - */
>> - if (enc)
>> - return tdx_enc_status_changed(vaddr, numpages, enc);
>> - return true;
>> -}
>> -
>> -static bool tdx_enc_status_change_finish(unsigned long vaddr, int
>> numpages,
>> - bool enc)
>> -{
>> - /*
>> - * Only handle private->shared conversion here.
>> - * See the comment in tdx_early_init().
>> - */
>> - if (!enc)
>> - return tdx_enc_status_changed(vaddr, numpages, enc);
>> - return true;
>> -}
>> -
>> void __init tdx_early_init(void)
>> {
>> struct tdx_module_args args = {
>> @@ -831,30 +789,8 @@ void __init tdx_early_init(void)
>> */
>> physical_mask &= cc_mask - 1;
>> - /*
>> - * The kernel mapping should match the TDX metadata for the page.
>> - * load_unaligned_zeropad() can touch memory *adjacent* to that
>> which is
>> - * owned by the caller and can catch even _momentary_ mismatches. Bad
>> - * things happen on mismatch:
>> - *
>> - * - Private mapping => Shared Page == Guest shutdown
>> - * - Shared mapping => Private Page == Recoverable #VE
>> - *
>> - * guest.enc_status_change_prepare() converts the page from
>> - * shared=>private before the mapping becomes private.
>> - *
>> - * guest.enc_status_change_finish() converts the page from
>> - * private=>shared after the mapping becomes private.
>> - *
>> - * In both cases there is a temporary shared mapping to a private
>> page,
>> - * which can result in a #VE. But, there is never a private
>> mapping to
>> - * a shared page.
>> - */
>> - x86_platform.guest.enc_status_change_prepare =
>> tdx_enc_status_change_prepare;
>> - x86_platform.guest.enc_status_change_finish =
>> tdx_enc_status_change_finish;
>> -
>> + x86_platform.guest.enc_status_change_finish = tdx_enc_status_changed;
>> x86_platform.guest.enc_cache_flush_required =
>> tdx_cache_flush_required;
>> - x86_platform.guest.enc_tlb_flush_required = tdx_tlb_flush_required;
>> /*
>> * TDX intercepts the RDMSR to read the X2APIC ID in the parallel
>> diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
>> index 084fab6..fbe2585 100644
>> --- a/arch/x86/hyperv/ivm.c
>> +++ b/arch/x86/hyperv/ivm.c
>> @@ -550,11 +550,6 @@ static bool hv_vtom_set_host_visibility(unsigned
>> long kbuffer, int pagecount, bo
>> return result;
>> }
>> -static bool hv_vtom_tlb_flush_required(bool private)
>> -{
>> - return true;
>> -}
>> -
>> static bool hv_vtom_cache_flush_required(void)
>> {
>> return false;
>> @@ -614,7 +609,6 @@ void __init hv_vtom_init(void)
>> x86_platform.hyper.is_private_mmio = hv_is_private_mmio;
>> x86_platform.guest.enc_cache_flush_required =
>> hv_vtom_cache_flush_required;
>> - x86_platform.guest.enc_tlb_flush_required =
>> hv_vtom_tlb_flush_required;
>> x86_platform.guest.enc_status_change_finish =
>> hv_vtom_set_host_visibility;
>> /* Set WB as the default cache mode. */
>> diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
>> index a37ebd3..cf5179b 100644
>> --- a/arch/x86/kernel/x86_init.c
>> +++ b/arch/x86/kernel/x86_init.c
>> @@ -131,9 +131,7 @@ struct x86_cpuinit_ops x86_cpuinit = {
>> static void default_nmi_init(void) { };
>> -static bool enc_status_change_prepare_noop(unsigned long vaddr, int
>> npages, bool enc) { return true; }
>> static bool enc_status_change_finish_noop(unsigned long vaddr, int
>> npages, bool enc) { return true; }
>> -static bool enc_tlb_flush_required_noop(bool enc) { return false; }
>> static bool enc_cache_flush_required_noop(void) { return false; }
>> static bool is_private_mmio_noop(u64 addr) {return false; }
>> @@ -154,9 +152,7 @@ struct x86_platform_ops x86_platform __ro_after_init
>> = {
>> .hyper.is_private_mmio = is_private_mmio_noop,
>> .guest = {
>> - .enc_status_change_prepare = enc_status_change_prepare_noop,
>> .enc_status_change_finish = enc_status_change_finish_noop,
>> - .enc_tlb_flush_required = enc_tlb_flush_required_noop,
>> .enc_cache_flush_required = enc_cache_flush_required_noop,
>> },
>> };
>> diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
>> index 6faea41..06960ba 100644
>> --- a/arch/x86/mm/mem_encrypt_amd.c
>> +++ b/arch/x86/mm/mem_encrypt_amd.c
>> @@ -278,11 +278,6 @@ static unsigned long pg_level_to_pfn(int level,
>> pte_t *kpte, pgprot_t *ret_prot)
>> return pfn;
>> }
>> -static bool amd_enc_tlb_flush_required(bool enc)
>> -{
>> - return true;
>> -}
>> -
>> static bool amd_enc_cache_flush_required(void)
>> {
>> return !cpu_feature_enabled(X86_FEATURE_SME_COHERENT);
>> @@ -318,18 +313,6 @@ static void enc_dec_hypercall(unsigned long vaddr,
>> unsigned long size, bool enc)
>> #endif
>> }
>> -static bool amd_enc_status_change_prepare(unsigned long vaddr, int
>> npages, bool enc)
>> -{
>> - /*
>> - * To maintain the security guarantees of SEV-SNP guests, make sure
>> - * to invalidate the memory before encryption attribute is cleared.
>> - */
>> - if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP) && !enc)
>> - snp_set_memory_shared(vaddr, npages);
>> -
>> - return true;
>> -}
>> -
>> /* Return true unconditionally: return value doesn't matter for the
>> SEV side */
>> static bool amd_enc_status_change_finish(unsigned long vaddr, int
>> npages, bool enc)
>> {
>> @@ -337,8 +320,12 @@ static bool amd_enc_status_change_finish(unsigned
>> long vaddr, int npages, bool e
>> * After memory is mapped encrypted in the page table, validate it
>> * so that it is consistent with the page table updates.
>> */
>> - if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP) && enc)
>> - snp_set_memory_private(vaddr, npages);
>> + if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP)) {
>> + if (enc)
>> + snp_set_memory_private(vaddr, npages);
>> + else
>> + snp_set_memory_shared(vaddr, npages);
>> + }
>
> These calls will both result in a PVALIDATE being issued (either before or
> after the page state change to the hypervisor) using the virtual address,
> which will trigger a #PF is the present bit isn't set.
>
>> if (!cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT))
>> enc_dec_hypercall(vaddr, npages << PAGE_SHIFT, enc);
>> @@ -498,9 +485,7 @@ void __init sme_early_init(void)
>> /* Update the protection map with memory encryption mask */
>> add_encrypt_protection_map();
>> - x86_platform.guest.enc_status_change_prepare =
>> amd_enc_status_change_prepare;
>> x86_platform.guest.enc_status_change_finish =
>> amd_enc_status_change_finish;
>> - x86_platform.guest.enc_tlb_flush_required =
>> amd_enc_tlb_flush_required;
>> x86_platform.guest.enc_cache_flush_required =
>> amd_enc_cache_flush_required;
>> /*
>> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
>> index d7ef8d3..d062e01 100644
>> --- a/arch/x86/mm/pat/set_memory.c
>> +++ b/arch/x86/mm/pat/set_memory.c
>> @@ -2147,40 +2147,57 @@ static int __set_memory_enc_pgtable(unsigned
>> long addr, int numpages, bool enc)
>> memset(&cpa, 0, sizeof(cpa));
>> cpa.vaddr = &addr;
>> cpa.numpages = numpages;
>> +
>> + /*
>> + * The caller must ensure that the memory being transitioned between
>> + * encrypted and decrypted is not being accessed. But if
>> + * load_unaligned_zeropad() touches the "next" page, it may generate a
>> + * read access the caller has no control over. To ensure such accesses
>> + * cause a normal page fault for the load_unaligned_zeropad() handler,
>> + * mark the pages not present until the transition is complete. We
>> + * don't want a #VE or #VC fault due to a mismatch in the memory
>> + * encryption status, since paravisor configurations can't cleanly do
>> + * the load_unaligned_zeropad() handling in the paravisor.
>> + *
>> + * There's no requirement to do so, but for efficiency we can clear
>> + * _PAGE_PRESENT and set/clr encryption attr as a single operation.
>> + */
>> cpa.mask_set = enc ? pgprot_encrypted(empty) :
>> pgprot_decrypted(empty);
>> - cpa.mask_clr = enc ? pgprot_decrypted(empty) :
>> pgprot_encrypted(empty);
>> + cpa.mask_clr = enc ? pgprot_decrypted(__pgprot(_PAGE_PRESENT)) :
>> + pgprot_encrypted(__pgprot(_PAGE_PRESENT));
>
> This should be lined up with the pgprot_decrypted above, e.g.:
>
> cpa.mask_clr = enc ? pgprot_decrypted(__pgprot(_PAGE_PRESENT)) :
> pgprot_encrypted(__pgprot(_PAGE_PRESENT));
>
> or
>
> cpa.mask_clr = enc ? pgprot_decrypted(__pgprot(_PAGE_PRESENT))
> : pgprot_encrypted(__pgprot(_PAGE_PRESENT));
>
>> cpa.pgd = init_mm.pgd;
>> /* Must avoid aliasing mappings in the highmem code */
>> kmap_flush_unused();
>> vm_unmap_aliases();
>> - /* Flush the caches as needed before changing the encryption
>> attribute. */
>> - if (x86_platform.guest.enc_tlb_flush_required(enc))
>> - cpa_flush(&cpa, x86_platform.guest.enc_cache_flush_required());
>> -
>> - /* Notify hypervisor that we are about to set/clr encryption
>> attribute. */
>> - if (!x86_platform.guest.enc_status_change_prepare(addr, numpages,
>> enc))
>> - return -EIO;
>> + /* Flush the caches as needed before changing the encryption attr. */
>> + if (x86_platform.guest.enc_cache_flush_required())
>> + cpa_flush(&cpa, 1);
>> ret = __change_page_attr_set_clr(&cpa, 1);
>> + if (ret)
>> + return ret;
>> /*
>> - * After changing the encryption attribute, we need to flush TLBs
>> again
>> - * in case any speculative TLB caching occurred (but no need to flush
>> - * caches again). We could just use cpa_flush_all(), but in case TLB
>> - * flushing gets optimized in the cpa_flush() path use the same logic
>> - * as above.
>> + * After clearing _PAGE_PRESENT and changing the encryption attribute,
>> + * we need to flush TLBs to ensure no further accesses to the
>> memory can
>> + * be made with the old encryption attribute (but no need to flush
>> caches
>> + * again). We could just use cpa_flush_all(), but in case TLB
>> flushing
>> + * gets optimized in the cpa_flush() path use the same logic as above.
>> */
>> cpa_flush(&cpa, 0);
>> - /* Notify hypervisor that we have successfully set/clr encryption
>> attribute. */
>> - if (!ret) {
>> - if (!x86_platform.guest.enc_status_change_finish(addr,
>> numpages, enc))
>> - ret = -EIO;
>> - }
>> + /* Notify hypervisor that we have successfully set/clr encryption
>> attr. */
>> + if (!x86_platform.guest.enc_status_change_finish(addr, numpages, enc))
>> + return -EIO;
>
> Here's where the #PF is likely to be triggered.
>
> Thanks,
> Tom
>
>> - return ret;
>> + /*
>> + * Now that the hypervisor is sync'ed with the page table changes
>> + * made here, add back _PAGE_PRESENT. set_memory_p() does not flush
>> + * the TLB.
>> + */
>> + return set_memory_p(&addr, numpages);
>> }
>> static int __set_memory_enc_dec(unsigned long addr, int numpages, bool
>> enc)
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