| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <a940044f-3ae4-451f-b9ba-946ec6df5082@amazon.com> Date: Fri, 7 Nov 2025 18:11:01 +0000 From: Nikita Kalyazin <kalyazin@...zon.com> To: 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 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 > > Anyway, the solution I'm hoping to present for your problem gets rid of > that explicit unmapping code (the allocator will do it for you), so in > the meantime I have something I can work with.
Powered by blists - more mailing lists