| Message ID | 20231121212016.1154303-1-mhklinux@outlook.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | x86/coco: Mark CoCo VM pages not present when changing encrypted state | expand |
On Tue, Nov 21, 2023 at 01:20:08PM -0800, mhkelley58@gmail.com wrote: > From: Michael Kelley <mhklinux@outlook.com> > > 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. Strictly speaking it is not true for TDX. Conversion to shared can be implicit: set shared bit and touch the page will do the conversion. MapGPA is optional. > 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 read 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) > if the exceptions are routed to the paravisor. The paravisor can't > do load_unaligned_zeropad() fixup, so the exceptions would need to > be forwarded from the paravisor to the Linux guest, but there are > no architectural specs for how to do that. Hm. Can't we inject #PF (or #GP) into L2 if #VE/#VC handler in L1 sees cross-page access to shared memory while no fixup entry for the page in L1. It would give L2 chance to handle the situation in a transparent way. Maybe I miss something, I donno.
From: kirill.shutemov@linux.intel.com <kirill.shutemov@linux.intel.com> Sent: Friday, November 24, 2023 2:06 AM > > On Tue, Nov 21, 2023 at 01:20:08PM -0800, mhkelley58@gmail.com wrote: > > From: Michael Kelley <mhklinux@outlook.com> > > > > 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. > > Strictly speaking it is not true for TDX. Conversion to shared can be > implicit: set shared bit and touch the page will do the conversion. MapGPA > is optional. Interesting. Given that, is there a reason to use the explicit hypervisor callbacks in for private->shared transitions in __set_mem_enc_pgtable()? It probably doesn't have direct relevance to this patch series, but I'm just trying to understand the tradeoffs of the implicit vs. explicit approach. And am I correct that shared->private transitions must use the explicit approach? > > > 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 read 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) > > if the exceptions are routed to the paravisor. The paravisor can't > > do load_unaligned_zeropad() fixup, so the exceptions would need to > > be forwarded from the paravisor to the Linux guest, but there are > > no architectural specs for how to do that. > > Hm. Can't we inject #PF (or #GP) into L2 if #VE/#VC handler in L1 sees > cross-page access to shared memory while no fixup entry for the page in > L1. It would give L2 chance to handle the situation in a transparent way. > > Maybe I miss something, I donno. I'm recounting what the Hyper-V paravisor folks say without knowing all the details. :-( But it seems like any kind of forwarding scheme needs to be a well-defined contract that would work for both TDX and SEV-SNP. The paravisor in L1 might or might not be Linux-based, so the contract must be OS independent. And the L2 guest might or might not be Linux, so there's potential for some other kind of error to be confused with a Linux load_unaligned_zeropad() reference. Maybe all that could be sorted out, but I come back to believing that it's better now and in the long run to just avoid all this complexity by decoupling private-shared page transitions and Linux load_unaligned_zeropad(). Unfortunately that decoupling hasn't been as simple as I first envisioned because of SEV-SNP PVALIDATE needing a virtual address. But doing the decoupling only in the paravisor case still seems like the simpler approach. Michael > > -- > Kiryl Shutsemau / Kirill A. Shutemov
On Tue, Nov 28, 2023 at 07:12:33PM +0000, Michael Kelley wrote: > From: kirill.shutemov@linux.intel.com <kirill.shutemov@linux.intel.com> Sent: Friday, November 24, 2023 2:06 AM > > > > On Tue, Nov 21, 2023 at 01:20:08PM -0800, mhkelley58@gmail.com wrote: > > > From: Michael Kelley <mhklinux@outlook.com> > > > > > > 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. > > > > Strictly speaking it is not true for TDX. Conversion to shared can be > > implicit: set shared bit and touch the page will do the conversion. MapGPA > > is optional. > > Interesting. Given that, is there a reason to use the explicit > hypervisor callbacks in for private->shared transitions in > __set_mem_enc_pgtable()? It probably doesn't have direct relevance > to this patch series, but I'm just trying to understand the tradeoffs of > the implicit vs. explicit approach. And am I correct that > shared->private transitions must use the explicit approach? It must be explicit in sense, that the memory has to be accepted before use. MapGPA() is still optional. I don't like this implicit tricks. I spent a lot of time debugging an issue that was obscured by this semantics. But I think it is going to say :/ > > > 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 read 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) > > > if the exceptions are routed to the paravisor. The paravisor can't > > > do load_unaligned_zeropad() fixup, so the exceptions would need to > > > be forwarded from the paravisor to the Linux guest, but there are > > > no architectural specs for how to do that. > > > > Hm. Can't we inject #PF (or #GP) into L2 if #VE/#VC handler in L1 sees > > cross-page access to shared memory while no fixup entry for the page in > > L1. It would give L2 chance to handle the situation in a transparent way. > > > > Maybe I miss something, I donno. > > I'm recounting what the Hyper-V paravisor folks say without knowing all the > details. :-( But it seems like any kind of forwarding scheme needs to be a > well-defined contract that would work for both TDX and SEV-SNP. The > paravisor in L1 might or might not be Linux-based, so the contract must be OS > independent. And the L2 guest might or might not be Linux, so there's > potential for some other kind of error to be confused with a Linux > load_unaligned_zeropad() reference. Okay, fair enough. I have hard time reasoning if it is okay for L2 which is not Linux.
From: Michael Kelley <mhklinux@outlook.com> 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 read 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) if the exceptions are routed to the paravisor. The paravisor can't do load_unaligned_zeropad() fixup, so the exceptions would 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 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. Patch 1 handles implications of the hypervisor callbacks in Step 6 needing to do virt-to-phys translations on pages that are temporarily marked not present. Patch 2 ensures that Step 7 doesn't generate a TLB flush. It is a performance optimization only and is not necessary for correctness. Patches 3 and 4 handle the case where SEV-SNP does PVALIDATE in Step 6, which requires a valid virtual address. But since the PRESENT bit has been removed from the direct map virtual address, the PVALIDATE fails. These patches construct a temporary virtual address to be used by PVALIDATE. This code is SEV-SNP only because the TDX and Hyper-V paravisor flavors operate on physical addresses. Patches 5 and 6 are the core change that marks the transitioning pages as not present. Patch 6 is optional since retaining both the "prepare" and "finish" callbacks doesn't hurt anything and there might be an argument for retaining both for future flexibility. However, Patch 6 *does* eliminate about 75 lines of code and comments. Patch 7 is a somewhat tangential cleanup that removes an unnecessary wrapper function in the path for doing a transition. Patch 8 adds comments describing the implications of errors when doing a transition. These implications are discussed in the email thread for the RFC patch[1] and a patch proposed by Rick Edgecombe. [2][3] With this change, the #VE and #VC exception handlers should no longer be triggered for load_unaligned_zeropad() accesses, and the existing code in those handlers to do the "fixup" shouldn't be needed. But I have not removed that code in this patch set. Kirill Shutemov wants to keep the code for TDX #VE, so the code for #VC on the SEV-SNP side has also been kept. This patch set is based on the linux-next20231117 code tree. Changes in v2: * Added Patches 3 and 4 to deal with the failure on SEV-SNP [Tom Lendacky] * Split the main change into two separate patches (Patch 5 and Patch 6) to improve reviewability and to offer the option of retaining both hypervisor callbacks. * Patch 5 moves set_memory_p() out of an #ifdef CONFIG_X86_64 so that the code builds correctly for 32-bit, even though it is never executed for 32-bit [reported by kernel test robot] [1] https://lore.kernel.org/lkml/1688661719-60329-1-git-send-email-mikelley@microsoft.com/ [2] https://lore.kernel.org/lkml/20231017202505.340906-1-rick.p.edgecombe@intel.com/ [3] https://lore.kernel.org/lkml/20231024234829.1443125-1-rick.p.edgecombe@intel.com/ Michael Kelley (8): x86/coco: Use slow_virt_to_phys() in page transition hypervisor callbacks x86/mm: Don't do a TLB flush if changing a PTE that isn't marked present x86/mm: Remove "static" from vmap_pages_range() x86/sev: Enable PVALIDATE for PFNs without a valid virtual address x86/mm: Mark CoCo VM pages not present while changing encrypted state x86/mm: Merge CoCo prepare and finish hypervisor callbacks x86/mm: Remove unnecessary call layer for __set_memory_enc_pgtable() x86/mm: Add comments about errors in set_memory_decrypted()/encrypted() arch/x86/boot/compressed/sev.c | 2 +- arch/x86/coco/tdx/tdx.c | 66 +---------------- arch/x86/hyperv/ivm.c | 15 ++-- arch/x86/include/asm/sev.h | 6 +- arch/x86/include/asm/x86_init.h | 4 -- arch/x86/kernel/sev-shared.c | 57 ++++++++++++--- arch/x86/kernel/sev.c | 43 ++++++----- arch/x86/kernel/x86_init.c | 4 -- arch/x86/mm/mem_encrypt_amd.c | 23 +----- arch/x86/mm/pat/set_memory.c | 122 ++++++++++++++++++++++---------- include/linux/vmalloc.h | 2 + mm/vmalloc.c | 2 +- 12 files changed, 171 insertions(+), 175 deletions(-)