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Message-ID: <DE54JGHVJ4QH.KW6CD73R58AU@google.com> Date: Mon, 10 Nov 2025 15:36:17 +0000 From: Brendan Jackman <jackmanb@...gle.com> To: <kalyazin@...zon.com>, Brendan Jackman <jackmanb@...gle.com> Cc: <pbonzini@...hat.com>, <corbet@....net>, <maz@...nel.org>, <oliver.upton@...ux.dev>, <joey.gouly@....com>, <suzuki.poulose@....com>, <yuzenghui@...wei.com>, <catalin.marinas@....com>, <will@...nel.org>, <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>, <x86@...nel.org>, <hpa@...or.com>, <luto@...nel.org>, <peterz@...radead.org>, <willy@...radead.org>, <akpm@...ux-foundation.org>, <david@...hat.com>, <lorenzo.stoakes@...cle.com>, <Liam.Howlett@...cle.com>, <vbabka@...e.cz>, <rppt@...nel.org>, <surenb@...gle.com>, <mhocko@...e.com>, <song@...nel.org>, <jolsa@...nel.org>, <ast@...nel.org>, <daniel@...earbox.net>, <andrii@...nel.org>, <martin.lau@...ux.dev>, <eddyz87@...il.com>, <yonghong.song@...ux.dev>, <john.fastabend@...il.com>, <kpsingh@...nel.org>, <sdf@...ichev.me>, <haoluo@...gle.com>, <jgg@...pe.ca>, <jhubbard@...dia.com>, <peterx@...hat.com>, <jannh@...gle.com>, <pfalcato@...e.de>, <shuah@...nel.org>, <seanjc@...gle.com>, <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>, <kvmarm@...ts.linux.dev>, <linux-fsdevel@...r.kernel.org>, <linux-mm@...ck.org>, <bpf@...r.kernel.org>, <linux-kselftest@...r.kernel.org>, <xmarcalx@...zon.co.uk>, <kalyazin@...zon.co.uk>, <jackabt@...zon.co.uk>, <derekmn@...zon.co.uk>, <tabba@...gle.com>, <ackerleytng@...gle.com>, Patrick Roy <patrick.roy@...pus.lmu.de> Subject: Re: [PATCH v7 00/12] Direct Map Removal Support for guest_memfd On Fri Nov 7, 2025 at 6:11 PM UTC, Nikita Kalyazin wrote: > > > On 07/11/2025 18:04, Brendan Jackman wrote: >> On Fri Nov 7, 2025 at 5:23 PM UTC, Nikita Kalyazin wrote: >>> >>> >>> On 07/11/2025 15:54, Brendan Jackman wrote: >>>> On Wed Sep 24, 2025 at 3:10 PM UTC, Patrick Roy wrote: >>>>> From: Patrick Roy <roypat@...zon.co.uk> >>>>> >>>>> [ based on kvm/next ] >>>>> >>>>> Unmapping virtual machine guest memory from the host kernel's direct map is a >>>>> successful mitigation against Spectre-style transient execution issues: If the >>>>> kernel page tables do not contain entries pointing to guest memory, then any >>>>> attempted speculative read through the direct map will necessarily be blocked >>>>> by the MMU before any observable microarchitectural side-effects happen. This >>>>> means that Spectre-gadgets and similar cannot be used to target virtual machine >>>>> memory. Roughly 60% of speculative execution issues fall into this category [1, >>>>> Table 1]. >>>>> >>>>> This patch series extends guest_memfd with the ability to remove its memory >>>>> from the host kernel's direct map, to be able to attain the above protection >>>>> for KVM guests running inside guest_memfd. >>>>> >>>>> Additionally, a Firecracker branch with support for these VMs can be found on >>>>> GitHub [2]. >>>>> >>>>> For more details, please refer to the v5 cover letter [v5]. No >>>>> substantial changes in design have taken place since. >>>>> >>>>> === Changes Since v6 === >>>>> >>>>> - Drop patch for passing struct address_space to ->free_folio(), due to >>>>> possible races with freeing of the address_space. (Hugh) >>>>> - Stop using PG_uptodate / gmem preparedness tracking to keep track of >>>>> direct map state. Instead, use the lowest bit of folio->private. (Mike, David) >>>>> - Do direct map removal when establishing mapping of gmem folio instead >>>>> of at allocation time, due to impossibility of handling direct map >>>>> removal errors in kvm_gmem_populate(). (Patrick) >>>>> - Do TLB flushes after direct map removal, and provide a module >>>>> parameter to opt out from them, and a new patch to export >>>>> flush_tlb_kernel_range() to KVM. (Will) >>>>> >>>>> [1]: https://download.vusec.net/papers/quarantine_raid23.pdf >>>>> [2]: https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding >>>> >>>> I just got around to trying this out, I checked out this patchset using >>>> its base-commit and grabbed the Firecracker branch. Things seem OK until >>>> I set the secrets_free flag in the Firecracker config which IIUC makes >>>> it set GUEST_MEMFD_FLAG_NO_DIRECT_MAP. >>>> >>>> If I set it, I find the guest doesn't show anything on the console. >>>> Running it in a VM and attaching GDB suggests that it's entering the >>>> guest repeatedly, it doesn't seem like the vCPU thread is stuck or >>>> anything. I'm a bit clueless about how to debug that (so far, whenever >>>> I've broken KVM, things always exploded very dramatically). >>>> >>>> Anyway, if I then kill the firecracker process, the host sometimes >>>> crashes, I think this is the most suggestive splat I've seen: >>>> >>>> [ 99.673420][ T2] BUG: unable to handle page fault for address: ffff888012804000 >>>> [ 99.676216][ T2] #PF: supervisor write access in kernel mode >>>> [ 99.678381][ T2] #PF: error_code(0x0002) - not-present page >>>> [ 99.680499][ T2] PGD 2e01067 P4D 2e01067 PUD 2e02067 PMD 12801063 PTE 800fffffed7fb020 >>>> [ 99.683374][ T2] Oops: Oops: 0002 [#1] SMP >>>> [ 99.685004][ T2] CPU: 0 UID: 0 PID: 2 Comm: kthreadd Not tainted 6.17.0-rc7-00366-g473c46a3cb2a #106 NONE >>>> [ 99.688514][ T2] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.1 11/11/2019 >>>> [ 99.691547][ T2] RIP: 0010:clear_page_erms+0x7/0x10 >>>> [ 99.693440][ T2] Code: 48 89 47 18 48 89 47 20 48 89 47 28 48 89 47 30 48 89 47 38 48 8d 7f 40 75 d9 90 c3 0f 1f 80 00 00 00 00 b9 00 10 00 00 31 c0 <f3> aa c3 66 0f 1f 44 00 00 48 83 f9 40 73 2a 83 f9 08 73 0f 85 c9 >>>> [ 99.700188][ T2] RSP: 0018:ffff88800318fc10 EFLAGS: 00010246 >>>> [ 99.702321][ T2] RAX: 0000000000000000 RBX: 0000000000400dc0 RCX: 0000000000001000 >>>> [ 99.705100][ T2] RDX: ffffea00004a0100 RSI: ffffea00004a0200 RDI: ffff888012804000 >>>> [ 99.707861][ T2] RBP: 0000000000000801 R08: 0000000000000000 R09: 0000000000000000 >>>> [ 99.710648][ T2] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000002 >>>> [ 99.713412][ T2] R13: 0000000000000801 R14: ffffea00004a0100 R15: ffffffff81f4df80 >>>> [ 99.716191][ T2] FS: 0000000000000000(0000) GS:ffff8880bbf28000(0000) knlGS:0000000000000000 >>>> [ 99.719316][ T2] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>> [ 99.721648][ T2] CR2: ffff888012804000 CR3: 0000000007583001 CR4: 0000000000372eb0 >>>> [ 99.724421][ T2] Call Trace: >>>> [ 99.725608][ T2] <TASK> >>>> [ 99.726646][ T2] get_page_from_freelist+0x6fe/0x14b0 >>>> [ 99.728583][ T2] ? fs_reclaim_acquire+0x43/0xe0 >>>> [ 99.730325][ T2] ? find_held_lock+0x2b/0x80 >>>> [ 99.731965][ T2] __alloc_frozen_pages_noprof+0x147/0x2d0 >>>> [ 99.734003][ T2] __alloc_pages_noprof+0x5/0x50 >>>> [ 99.735766][ T2] copy_process+0x1b1/0x1b30 >>>> [ 99.737398][ T2] ? lock_is_held_type+0x89/0x100 >>>> [ 99.739157][ T2] ? kthreadd+0x25/0x190 >>>> [ 99.740664][ T2] kernel_clone+0x59/0x390 >>>> [ 99.742213][ T2] ? kthreadd+0x25/0x190 >>>> [ 99.743728][ T2] kernel_thread+0x55/0x70 >>>> [ 99.745310][ T2] ? kthread_complete_and_exit+0x20/0x20 >>>> [ 99.747265][ T2] kthreadd+0x117/0x190 >>>> [ 99.748748][ T2] ? kthread_is_per_cpu+0x30/0x30 >>>> [ 99.750509][ T2] ret_from_fork+0x16b/0x1e0 >>>> [ 99.752193][ T2] ? kthread_is_per_cpu+0x30/0x30 >>>> [ 99.753992][ T2] ret_from_fork_asm+0x11/0x20 >>>> [ 99.755717][ T2] </TASK> >>>> [ 99.756861][ T2] CR2: ffff888012804000 >>>> [ 99.758353][ T2] ---[ end trace 0000000000000000 ]--- >>>> [ 99.760319][ T2] RIP: 0010:clear_page_erms+0x7/0x10 >>>> [ 99.762209][ T2] Code: 48 89 47 18 48 89 47 20 48 89 47 28 48 89 47 30 48 89 47 38 48 8d 7f 40 75 d9 90 c3 0f 1f 80 00 00 00 00 b9 00 10 00 00 31 c0 <f3> aa c3 66 0f 1f 44 00 00 48 83 f9 40 73 2a 83 f9 08 73 0f 85 c9 >>>> [ 99.769129][ T2] RSP: 0018:ffff88800318fc10 EFLAGS: 00010246 >>>> [ 99.771297][ T2] RAX: 0000000000000000 RBX: 0000000000400dc0 RCX: 0000000000001000 >>>> [ 99.774126][ T2] RDX: ffffea00004a0100 RSI: ffffea00004a0200 RDI: ffff888012804000 >>>> [ 99.777013][ T2] RBP: 0000000000000801 R08: 0000000000000000 R09: 0000000000000000 >>>> [ 99.779827][ T2] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000002 >>>> [ 99.782641][ T2] R13: 0000000000000801 R14: ffffea00004a0100 R15: ffffffff81f4df80 >>>> [ 99.785487][ T2] FS: 0000000000000000(0000) GS:ffff8880bbf28000(0000) knlGS:0000000000000000 >>>> [ 99.788671][ T2] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>> [ 99.791012][ T2] CR2: ffff888012804000 CR3: 0000000007583001 CR4: 0000000000372eb0 >>>> [ 99.793863][ T2] Kernel panic - not syncing: Fatal exception >>>> [ 99.796760][ T2] Kernel Offset: disabled >>>> [ 99.798296][ T2] ---[ end Kernel panic - not syncing: Fatal exception ]--- >>>> >>>> This makes me suspect the kvm_gmem_folio_restore_direct_map() path isn't >>>> working or isn't getting called. >>>> >>>> If anyone wants help trying to reproduce this let me know. >>> >>> Hi Brendan, >>> >>> Thanks for trying to run it! >>> >>> Just as a sanity check, the way it is known for us to work is we apply >>> all patches from [1]. For booted VMs (as opposed to restored from >>> snapshot), apart from the v6 of the direct map removal series, the only >>> additional patch is a fix for kvmclock on x86 [2]. Please let me know >>> if you see the same issue with that patch applied too. >>> >>> Nikita >>> >>> [1] >>> https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding/resources/hiding_ci/linux_patches >>> [2] >>> https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding/resources/hiding_ci/linux_patches/11-kvm-clock >> >> Ah, thanks! Seems I should have checked my inbox before sending my other >> mail. With the kvmclock fix applied to my host kernel, I start setting >> the other crash immediately when the VM boots. If I comment out the >> actual unmapping of memory, it boots (before, it wouldn't boot even with >> that commented out). >> >> For the other linux_patches, I couldn't apply them on top of this >> series, do you have a branch I can use as a reference? > > Instead of having an explicit branch, we apply all the patches on top of > [1]. There is a script that performs fetch/build/install end-to-end: [2]. > > [1] > https://github.com/firecracker-microvm/firecracker/blob/feature/secret-hiding/resources/hiding_ci/kernel_commit_hash > [2] > https://github.com/firecracker-microvm/firecracker/blob/feature/secret-hiding/resources/hiding_ci/build_and_install_kernel.sh Thanks, I was able to construct a branch and confirm the crashes go away. I guess this should block merging the feature though, right? Do you know which particular of the patches are the likely relevant ones here?
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